feat(export): 交付 Markdown → HTML 导出服务

实现 Export Service 完整生命周期：mistune AST → Document AST → HtmlExporter 渲染完整 HTML5，异步任务注册表 + 取消 + 24h 产物过期。新增 5 个 /api/exports 端点与 15 项测试；pdf/docx 与函数图像静态渲染留待后续 PR。

backend/app/export/__init__.py

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+"""Export Service：多格式文档导出（首批 HTML）。
+
+模块划分：
+- document.py        Document AST 内部协议 + DocumentExporter Protocol + ExportResult
+- markdown.py        mistune → Document AST 解析
+- exporters/html.py  HtmlExporter（Document AST → HTML5）
+- service.py         导出任务注册表、后台执行、取消与文件生命周期
+"""

backend/app/export/document.py

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+"""Document AST：导出器的内部中间表示（Internal Protocol，不放入 contracts.py）。
+
+契约 §10.3 规定节点用稳定判别字段 node_id / type / attributes / children / text，
+类型专有信息统一放 attributes（如 heading 的 level、link 的 href、image 的 src）。
+导出器据此递归渲染，对无法表示的节点记 warning，不静默丢弃。
+"""
+
+from __future__ import annotations
+
+from typing import Any, Protocol
+
+from pydantic import BaseModel, ConfigDict, Field
+
+from app.contracts import ExportOptions
+
+
+class DocumentNode(BaseModel):
+    """递归文档节点；type 取契约 §10.3 首批 node type 之一。"""
+
+    model_config = ConfigDict(extra="forbid")
+
+    type: str
+    node_id: str
+    attributes: dict[str, Any] = Field(default_factory=dict)
+    children: list["DocumentNode"] = Field(default_factory=list)
+    text: str = ""
+
+
+class Document(DocumentNode):
+    """根节点，type 固定为 document。"""
+
+    type: str = "document"
+
+
+class DocumentExporter(Protocol):
+    """导出器协议（契约 §10.3）：把 Document AST 渲染为指定格式的产物。"""
+
+    async def export(self, document: Document, options: ExportOptions) -> "ExportResult": ...
+
+
+class ExportResult(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+    content: bytes
+    mime_type: str
+    warnings: list[str] = Field(default_factory=list)

backend/app/export/exporters/__init__.py

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+"""Export 渲染器：Document AST → 具体格式产物。"""

backend/app/export/exporters/html.py

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+"""HtmlExporter：Document AST → 完整 HTML5 文档（内嵌基础 CSS）。
+
+对无法静态表达的节点（mermaid / function_plot）渲染为占位代码块并记 warning，不静默丢失；
+严重内容缺失由 service 层以 EXPORT_UNSUPPORTED_CONTENT 判定，本层只负责逐节点渲染。
+"""
+
+from __future__ import annotations
+
+import html
+from datetime import datetime
+
+from app.contracts import ExportOptions
+from app.export.document import Document, DocumentNode, ExportResult
+
+_MERMAID_WARNING = "mermaid 需前端渲染，已保留为占位代码块"
+_FUNCTION_PLOT_WARNING = "函数图像渲染将在后续版本提供，已保留为占位代码块"
+
+_BASE_CSS = """
+body { margin: 0; background: #f6f7f9; color: #1f2328; font: 15px/1.7 -apple-system, 'Segoe UI', 'Microsoft YaHei', sans-serif; }
+article { max-width: 860px; margin: 0 auto; padding: 40px 48px; background: #fff; }
+article.theme-dark { background: #0d1117; color: #c9d1d9; }
+h1, h2, h3, h4, h5, h6 { line-height: 1.3; margin: 1.4em 0 0.6em; }
+h1.title { margin-top: 0; }
+p { margin: 0.6em 0; }
+a { color: #0969da; }
+code { font-family: 'JetBrains Mono', Consolas, monospace; font-size: 0.9em; background: #f0f1f3; padding: 0.15em 0.35em; border-radius: 3px; }
+pre { background: #f6f8fa; padding: 14px 16px; border-radius: 6px; overflow-x: auto; }
+pre.code-theme-github-dark { background: #0d1117; color: #c9d1d9; }
+pre code { background: none; padding: 0; }
+pre.mermaid, pre.function-plot { border: 1px dashed #d0d7de; }
+blockquote { margin: 0.8em 0; padding: 0.2em 1em; border-left: 4px solid #d0d7de; color: #57606a; }
+img { max-width: 100%; }
+table { border-collapse: collapse; margin: 0.8em 0; }
+th, td { border: 1px solid #d0d7de; padding: 6px 12px; }
+th { background: #f6f8fa; }
+dl.metadata { font-size: 0.85em; color: #57606a; border-top: 1px solid #eaeef2; border-bottom: 1px solid #eaeef2; padding: 0.6em 0; }
+dl.metadata dt { display: inline; font-weight: 600; margin-right: 0.4em; }
+dl.metadata dd { display: inline; margin: 0 1.2em 0 0; }
+.math, .math-block { overflow-x: auto; padding: 0.4em 0; }
+.task-list-item { list-style: none; }
+.task-list-item input { margin-right: 0.4em; }
+hr { border: none; border-top: 1px solid #d0d7de; margin: 1.4em 0; }
+""".strip()
+
+
+class HtmlExporter:
+    """实现 DocumentExporter：递归渲染 Document AST 为完整 HTML5 文档。"""
+
+    async def export(self, document: Document, options: ExportOptions) -> ExportResult:
+        self._options = options
+        warnings: list[str] = []
+        body = self._render_children(document.children, warnings)
+        content = self._assemble(document, options, body, warnings)
+        return ExportResult(
+            content=content.encode("utf-8"), mime_type="text/html", warnings=warnings
+        )
+
+    def _assemble(
+        self, document: Document, options: ExportOptions, body: str, warnings: list[str]
+    ) -> str:
+        title = str(document.attributes.get("title") or "")
+        parts = [
+            "<!doctype html>",
+            '<html lang="zh-CN">',
+            "<head>",
+            '<meta charset="utf-8">',
+            '<meta name="viewport" content="width=device-width, initial-scale=1">',
+        ]
+        if title:
+            parts.append(f"<title>{html.escape(title)}</title>")
+        parts.append(f"<style>{_BASE_CSS}</style>")
+        parts.append("</head>")
+        parts.append("<body>")
+        parts.append(f'<article class="theme-{html.escape(options.theme_id)}">')
+        if options.include_title and title:
+            parts.append(f'<h1 class="title">{html.escape(title)}</h1>')
+        if options.include_metadata:
+            metadata = document.attributes.get("metadata")
+            if metadata:
+                parts.append(self._render_metadata(metadata))
+        parts.append(body)
+        parts.append("</article>")
+        parts.append("</body>")
+        parts.append("</html>")
+        return "\n".join(parts) + "\n"
+
+    def _render_metadata(self, metadata: dict) -> str:
+        entries = ["<dl", ' class="metadata">']
+        for key, value in metadata.items():
+            entries.append(f"<dt>{html.escape(str(key))}</dt>")
+            entries.append(f"<dd>{html.escape(self._fmt_meta_value(value))}</dd>")
+        entries.append("</dl>")
+        return "".join(entries)
+
+    @staticmethod
+    def _fmt_meta_value(value: object) -> str:
+        if isinstance(value, datetime):
+            return value.isoformat()
+        if isinstance(value, list):
+            return ", ".join(str(item) for item in value)
+        return str(value)
+
+    def _render_children(self, children: list[DocumentNode], warnings: list[str]) -> str:
+        return "".join(self._render_node(child, warnings) for child in children)
+
+    def _render_node(self, node: DocumentNode, warnings: list[str]) -> str:
+        handler = getattr(self, f"_render_{node.type}", None)
+        if handler is not None:
+            return handler(node, warnings)
+        warnings.append(f"无法表示的节点类型已跳过：{node.type}")
+        return ""
+
+    # --- 块级 ---
+    def _render_heading(self, node: DocumentNode, warnings: list[str]) -> str:
+        level = max(1, min(6, int(node.attributes.get("level", 1))))
+        return f"<h{level}>{self._render_children(node.children, warnings)}</h{level}>"
+
+    def _render_paragraph(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<p>{self._render_children(node.children, warnings)}</p>"
+
+    def _render_blockquote(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<blockquote>{self._render_children(node.children, warnings)}</blockquote>"
+
+    def _render_list(self, node: DocumentNode, warnings: list[str]) -> str:
+        tag = "ol" if node.attributes.get("ordered") else "ul"
+        return f"<{tag}>{self._render_children(node.children, warnings)}</{tag}>"
+
+    def _render_list_item(self, node: DocumentNode, warnings: list[str]) -> str:
+        inner = self._render_children(node.children, warnings)
+        if node.attributes.get("task"):
+            checked = " checked" if node.attributes.get("checked") else ""
+            return (
+                '<li class="task-list-item">'
+                f'<input type="checkbox" disabled{checked}>{inner}</li>'
+            )
+        return f"<li>{inner}</li>"
+
+    def _render_table(self, node: DocumentNode, warnings: list[str]) -> str:
+        rows = node.children
+        head_rows = [r for r in rows if r.attributes.get("head")]
+        body_rows = [r for r in rows if not r.attributes.get("head")]
+        parts = ["<table>"]
+        if head_rows:
+            parts.append("<thead>")
+            parts.extend(self._render_node(r, warnings) for r in head_rows)
+            parts.append("</thead>")
+        if body_rows:
+            parts.append("<tbody>")
+            parts.extend(self._render_node(r, warnings) for r in body_rows)
+            parts.append("</tbody>")
+        parts.append("</table>")
+        return "".join(parts)
+
+    def _render_table_row(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<tr>{self._render_children(node.children, warnings)}</tr>"
+
+    def _render_table_cell(self, node: DocumentNode, warnings: list[str]) -> str:
+        tag = "th" if node.attributes.get("head") else "td"
+        return f"<{tag}>{self._render_children(node.children, warnings)}</{tag}>"
+
+    def _render_code_block(self, node: DocumentNode, warnings: list[str]) -> str:
+        lang = str(node.attributes.get("language") or "")
+        code = html.escape(node.text)
+        lang_cls = f' class="language-{html.escape(lang)}"' if lang else ""
+        theme = html.escape(self._options.code_theme)
+        return f'<pre class="code-theme-{theme}"><code{lang_cls}>{code}</code></pre>'
+
+    def _render_thematic_break(self, node: DocumentNode, warnings: list[str]) -> str:
+        return "<hr>"
+
+    def _render_mermaid(self, node: DocumentNode, warnings: list[str]) -> str:
+        warnings.append(_MERMAID_WARNING)
+        return f'<pre class="mermaid">{html.escape(node.text)}</pre>'
+
+    def _render_function_plot(self, node: DocumentNode, warnings: list[str]) -> str:
+        warnings.append(_FUNCTION_PLOT_WARNING)
+        return f'<pre class="function-plot">{html.escape(node.text)}</pre>'
+
+    def _render_math_block(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f'<div class="math-block">$${html.escape(node.text)}$$</div>'
+
+    # --- 行内 ---
+    def _render_text(self, node: DocumentNode, warnings: list[str]) -> str:
+        return html.escape(node.text)
+
+    def _render_emphasis(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<em>{self._render_children(node.children, warnings)}</em>"
+
+    def _render_strong(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<strong>{self._render_children(node.children, warnings)}</strong>"
+
+    def _render_link(self, node: DocumentNode, warnings: list[str]) -> str:
+        href = html.escape(str(node.attributes.get("href") or ""))
+        title = str(node.attributes.get("title") or "")
+        attrs = [f'href="{href}"']
+        if title:
+            attrs.append(f'title="{html.escape(title)}"')
+        return f"<a {' '.join(attrs)}>{self._render_children(node.children, warnings)}</a>"
+
+    def _render_codespan(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"<code>{html.escape(node.text)}</code>"
+
+    def _render_image(self, node: DocumentNode, warnings: list[str]) -> str:
+        src = html.escape(str(node.attributes.get("src") or ""))
+        alt = html.escape(str(node.attributes.get("alt") or ""))
+        title = str(node.attributes.get("title") or "")
+        attrs = [f'src="{src}"', f'alt="{alt}"']
+        if title:
+            attrs.append(f'title="{html.escape(title)}"')
+        return f"<img {' '.join(attrs)}>"
+
+    def _render_math_inline(self, node: DocumentNode, warnings: list[str]) -> str:
+        return f"\\({html.escape(node.text)}\\)"
+
+    def _render_linebreak(self, node: DocumentNode, warnings: list[str]) -> str:
+        return "<br>"

backend/app/export/markdown.py

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+"""Markdown → Document AST：用 mistune 的 ast renderer 产出通用 token，再映射为内部节点。
+
+选用 mistune 内置 'ast' renderer 而非自写 BaseRenderer，是因为 mistune 的行内渲染按
+字符串拼接、无法承载结构化子节点；ast renderer 直接给出带 children/attrs/raw 的 token
+树，映射层只做 token → DocumentNode 的搬运，不掺入任何 HTML。
+"""
+
+from __future__ import annotations
+
+import mistune
+
+from app.export.document import Document, DocumentNode
+
+_PLUGINS = ["table", "math", "url", "task_lists"]
+
+# fenced code 语言分流：命中则转为专用节点，其余按普通代码块
+_MERMAID_LANG = "mermaid"
+_FUNCTION_PLOT_LANGS = {"function_plot", "functionplot"}
+
+
+def parse_document(markdown: str) -> Document:
+    """把 Markdown 文本解析为 Document AST 根节点。"""
+    renderer = mistune.create_markdown(renderer="ast", plugins=_PLUGINS)
+    tokens = renderer(markdown)
+    mapper = _AstMapper()
+    return Document(node_id=mapper.next_id(), children=mapper.map_blocks(tokens))
+
+
+class _AstMapper:
+    """token 树 → DocumentNode 树的映射器；node_id 按遍历顺序递增，无需跨请求稳定。"""
+
+    def __init__(self) -> None:
+        self._seq = 0
+
+    def next_id(self) -> str:
+        self._seq += 1
+        return f"node_{self._seq:03d}"
+
+    def map_blocks(self, tokens: list[dict]) -> list[DocumentNode]:
+        nodes: list[DocumentNode] = []
+        for token in tokens:
+            node = self.map_block(token)
+            if node is not None:
+                nodes.append(node)
+        return nodes
+
+    def map_block(self, token: dict) -> DocumentNode | None:
+        kind = token["type"]
+        if kind == "heading":
+            return DocumentNode(
+                type="heading",
+                node_id=self.next_id(),
+                attributes={"level": token["attrs"]["level"]},
+                children=self.map_inline(token.get("children", [])),
+            )
+        if kind in ("paragraph", "block_text"):
+            # block_text 是列表项内的段落块，仍按 paragraph 表达，由 list_item 包裹
+            return DocumentNode(
+                type="paragraph",
+                node_id=self.next_id(),
+                children=self.map_inline(token.get("children", [])),
+            )
+        if kind == "list":
+            return DocumentNode(
+                type="list",
+                node_id=self.next_id(),
+                attributes={"ordered": bool(token.get("attrs", {}).get("ordered"))},
+                children=[self.map_list_item(child) for child in token.get("children", [])],
+            )
+        if kind == "block_code":
+            return self._map_code(token)
+        if kind == "block_quote":
+            return DocumentNode(
+                type="blockquote",
+                node_id=self.next_id(),
+                children=self.map_blocks(token.get("children", [])),
+            )
+        if kind == "table":
+            return self._map_table(token)
+        if kind == "block_math":
+            return DocumentNode(
+                type="math_block", node_id=self.next_id(), text=token.get("raw", "")
+            )
+        if kind == "thematic_break":
+            return DocumentNode(type="thematic_break", node_id=self.next_id())
+        if kind == "blank_line":
+            return None
+        # 未知块级 token（如 block_html）保守保留原文，避免静默丢失
+        raw = token.get("raw", "")
+        if raw:
+            return DocumentNode(type="paragraph", node_id=self.next_id(), text=raw)
+        return None
+
+    def map_list_item(self, token: dict) -> DocumentNode:
+        """列表项：block_text 展平为行内子节点，嵌套 list 保留为子节点。"""
+        attributes: dict = {}
+        if token["type"] == "task_list_item":
+            attributes = {"task": True, "checked": bool(token.get("attrs", {}).get("checked"))}
+        children: list[DocumentNode] = []
+        for child in token.get("children", []):
+            if child["type"] == "block_text":
+                children.extend(self.map_inline(child.get("children", [])))
+            elif child["type"] == "list":
+                children.append(self.map_block(child))
+            else:
+                node = self.map_block(child)
+                if node is not None:
+                    children.append(node)
+        return DocumentNode(
+            type="list_item", node_id=self.next_id(), attributes=attributes, children=children
+        )
+
+    def map_inline(self, tokens: list[dict]) -> list[DocumentNode]:
+        nodes: list[DocumentNode] = []
+        for token in tokens:
+            node = self.map_inline_token(token)
+            if node is not None:
+                nodes.append(node)
+        return nodes
+
+    def map_inline_token(self, token: dict) -> DocumentNode | None:
+        kind = token["type"]
+        if kind == "text":
+            return DocumentNode(type="text", node_id=self.next_id(), text=token.get("raw", ""))
+        if kind == "strong":
+            return DocumentNode(
+                type="strong", node_id=self.next_id(),
+                children=self.map_inline(token.get("children", [])),
+            )
+        if kind == "emphasis":
+            return DocumentNode(
+                type="emphasis", node_id=self.next_id(),
+                children=self.map_inline(token.get("children", [])),
+            )
+        if kind == "link":
+            attrs = token.get("attrs", {})
+            attributes = {"href": attrs.get("url", "")}
+            if attrs.get("title"):
+                attributes["title"] = attrs["title"]
+            return DocumentNode(
+                type="link", node_id=self.next_id(), attributes=attributes,
+                children=self.map_inline(token.get("children", [])),
+            )
+        if kind == "codespan":
+            return DocumentNode(type="codespan", node_id=self.next_id(), text=token.get("raw", ""))
+        if kind == "image":
+            attrs = token.get("attrs", {})
+            attributes = {"src": attrs.get("src", "")}
+            if attrs.get("alt"):
+                attributes["alt"] = attrs["alt"]
+            if attrs.get("title"):
+                attributes["title"] = attrs["title"]
+            return DocumentNode(type="image", node_id=self.next_id(), attributes=attributes)
+        if kind == "inline_math":
+            return DocumentNode(
+                type="math_inline", node_id=self.next_id(), text=token.get("raw", "")
+            )
+        if kind == "softbreak":
+            # HTML 中换行会折叠为空白，软换行按空格表达
+            return DocumentNode(type="text", node_id=self.next_id(), text=" ")
+        if kind == "linebreak":
+            return DocumentNode(type="linebreak", node_id=self.next_id())
+        # 未知行内 token 保守保留原文
+        raw = token.get("raw", "")
+        if raw:
+            return DocumentNode(type="text", node_id=self.next_id(), text=raw)
+        return None
+
+    def _map_code(self, token: dict) -> DocumentNode:
+        info = (token.get("attrs", {}).get("info") or "").strip()
+        lang = info.split()[0].lower() if info else ""
+        code = token.get("raw", "").rstrip("\n")
+        if lang == _MERMAID_LANG:
+            return DocumentNode(type="mermaid", node_id=self.next_id(), text=code)
+        if lang in _FUNCTION_PLOT_LANGS:
+            return DocumentNode(type="function_plot", node_id=self.next_id(), text=code)
+        attributes = {"language": lang} if lang else {}
+        return DocumentNode(
+            type="code_block", node_id=self.next_id(), attributes=attributes, text=code
+        )
+
+    def _map_table(self, token: dict) -> DocumentNode:
+        rows: list[DocumentNode] = []
+        for child in token.get("children", []):
+            if child["type"] == "table_head":
+                rows.append(self._map_table_row(child, head=True))
+            elif child["type"] == "table_body":
+                for row in child.get("children", []):
+                    if row["type"] == "table_row":
+                        rows.append(self._map_table_row(row, head=False))
+            elif child["type"] == "table_row":
+                rows.append(self._map_table_row(child, head=False))
+        return DocumentNode(type="table", node_id=self.next_id(), children=rows)
+
+    def _map_table_row(self, token: dict, *, head: bool) -> DocumentNode:
+        cells: list[DocumentNode] = []
+        for cell in token.get("children", []):
+            if cell["type"] != "table_cell":
+                continue
+            attrs = cell.get("attrs", {})
+            cell_attributes = {"head": bool(attrs.get("head", head))}
+            if attrs.get("align"):
+                cell_attributes["align"] = attrs["align"]
+            cells.append(
+                DocumentNode(
+                    type="table_cell",
+                    node_id=self.next_id(),
+                    attributes=cell_attributes,
+                    children=self.map_inline(cell.get("children", [])),
+                )
+            )
+        return DocumentNode(
+            type="table_row", node_id=self.next_id(), attributes={"head": head}, children=cells
+        )

backend/app/export/service.py

@@ -0,0 +1,272 @@
+"""Export 服务：任务注册表、后台渲染、取消与产物生命周期。
+
+与 Benchmark 一致采用「创建即返回 queued、后台 Task 异步执行」的内存模型：任务与产物
+暂存内存与 exports 目录，不持久化到 SQLite。导出是单阶段渲染，无 SSE 事件流，取消主要
+在渲染前/后让出执行权的边界生效；产物带 24h 过期时间，过期后不可下载。
+"""
+
+from __future__ import annotations
+
+import asyncio
+import hashlib
+import logging
+import re
+from datetime import datetime, timedelta, timezone
+from pathlib import Path
+from uuid import uuid4
+
+from app.config import get_settings
+from app.contracts import (
+    ExportFile,
+    ExportFormat,
+    ExportJob,
+    ExportOptions,
+    ExportProgress,
+    ExportRequest,
+    ExportSource,
+    ExportSourceType,
+    ExportStatus,
+)
+from app.errors import ApiError
+from app.export.document import Document
+from app.export.exporters.html import HtmlExporter
+from app.export.markdown import parse_document
+from app.services import note_service
+
+logger = logging.getLogger(__name__)
+
+_jobs: dict[str, ExportJob] = {}
+_tasks: dict[str, asyncio.Task] = {}
+_cancel_flags: dict[str, asyncio.Event] = {}
+MAX_JOBS = 100
+# markdown 源大小上限，防止未保存预览塞爆内存/产物
+MAX_MARKDOWN_CHARS = 200_000
+# 产物有效期
+FILE_TTL = timedelta(hours=24)
+
+_INVALID_FILE_CHARS = re.compile(r'[\\/:*?"<>|]')
+
+
+class ExportCancelled(Exception):
+    """导出在渲染前被取消时抛出，用于标记 cancelled。"""
+
+
+def _now() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+def _safe_download_name(title: str) -> str:
+    """清洗标题得到安全的下载文件名；空标题回退到 export。"""
+    name = _INVALID_FILE_CHARS.sub("_", title).strip() or "export"
+    return name[:80]
+
+
+def _forget(job_id: str) -> None:
+    _jobs.pop(job_id, None)
+    _tasks.pop(job_id, None)
+    _cancel_flags.pop(job_id, None)
+
+
+def _evict_terminal() -> bool:
+    """超过容量时淘汰最旧的终态任务；全为活动任务无法淘汰时返回 False。"""
+    terminal = (ExportStatus.completed, ExportStatus.failed, ExportStatus.cancelled)
+    while len(_jobs) >= MAX_JOBS:
+        victim = next((jid for jid, job in _jobs.items() if job.status in terminal), None)
+        if victim is None:
+            return False
+        _forget(victim)
+    return True
+
+
+async def _resolve_source(source: ExportSource) -> tuple[str, str, dict | None]:
+    """把导出源解析为 (markdown, title, metadata)；metadata 仅 note 源提供。"""
+    if source.type == ExportSourceType.note:
+        note = await note_service.get_note(source.note_id)
+        if note is None:
+            raise ApiError(
+                404,
+                "EXPORT_SOURCE_NOT_FOUND",
+                "note not found",
+                {"note_id": source.note_id},
+            )
+        metadata = {
+            "file_path": note.file_path,
+            "tags": note.tags,
+            "created_at": note.created_at,
+            "updated_at": note.updated_at,
+        }
+        return note.markdown, note.title, metadata
+
+    markdown = source.markdown or ""
+    if not markdown.strip():
+        raise ApiError(400, "EXPORT_OPTIONS_INVALID", "markdown source must not be empty")
+    if len(markdown) > MAX_MARKDOWN_CHARS:
+        raise ApiError(
+            400,
+            "EXPORT_OPTIONS_INVALID",
+            f"markdown source exceeds {MAX_MARKDOWN_CHARS} characters",
+            {"size": len(markdown), "limit": MAX_MARKDOWN_CHARS},
+        )
+    return markdown, "", None
+
+
+async def create_export(request: ExportRequest) -> ExportJob:
+    """创建导出任务，立即返回 queued 的 ExportJob，由后台 Task 渲染。"""
+    if request.format != ExportFormat.html:
+        raise ApiError(
+            400,
+            "EXPORT_FORMAT_UNSUPPORTED",
+            "PDF/DOCX 暂未实现，当前仅支持 HTML",
+            {"format": request.format.value},
+        )
+    markdown, title, metadata = await _resolve_source(request.source)
+
+    if not _evict_terminal():
+        raise ApiError(
+            429,
+            "EXPORT_CAPACITY_EXCEEDED",
+            "Export capacity exceeded; wait for active jobs to finish.",
+            {},
+        )
+
+    job_id = "export_" + uuid4().hex[:12]
+    job = ExportJob(
+        job_id=job_id,
+        status=ExportStatus.queued,
+        format=request.format,
+        created_at=_now(),
+    )
+    _jobs[job_id] = job
+    _cancel_flags[job_id] = asyncio.Event()
+    _tasks[job_id] = asyncio.create_task(
+        _execute(job_id, markdown, title, metadata, request.options)
+    )
+    return job
+
+
+async def _execute(
+    job_id: str,
+    markdown: str,
+    title: str,
+    metadata: dict | None,
+    options: ExportOptions,
+) -> None:
+    """后台渲染：解析 → 导出 → 写文件 → 挂载产物元信息。"""
+    cancel_event = _cancel_flags[job_id]
+    _jobs[job_id] = _jobs[job_id].model_copy(
+        update={
+            "status": ExportStatus.running,
+            "started_at": _now(),
+            "progress": ExportProgress(phase="rendering", current=0, total=1, percent=0.0),
+        }
+    )
+    try:
+        # 让出一次，使「创建后立即取消」的 queued 任务能及时进入 cancelled
+        await asyncio.sleep(0)
+        if cancel_event.is_set():
+            raise ExportCancelled()
+
+        document = parse_document(markdown)
+        document.attributes["title"] = title
+        if metadata:
+            document.attributes["metadata"] = metadata
+
+        exporter = HtmlExporter()
+        result = await exporter.export(document, options)
+        if cancel_event.is_set():
+            raise ExportCancelled()
+
+        out_dir = get_settings().exports_path
+        out_dir.mkdir(parents=True, exist_ok=True)
+        path = out_dir / f"{job_id}.html"
+        path.write_bytes(result.content)
+
+        completed_at = _now()
+        _jobs[job_id] = _jobs[job_id].model_copy(
+            update={
+                "status": ExportStatus.completed,
+                "progress": ExportProgress(
+                    phase="completed", current=1, total=1, percent=1.0
+                ),
+                "file": ExportFile(
+                    file_name=f"{_safe_download_name(title)}.html",
+                    mime_type=result.mime_type,
+                    size=len(result.content),
+                    sha256=hashlib.sha256(result.content).hexdigest(),
+                    expires_at=completed_at + FILE_TTL,
+                ),
+                "warnings": result.warnings,
+                "completed_at": completed_at,
+            }
+        )
+    except ExportCancelled:
+        _jobs[job_id] = _jobs[job_id].model_copy(
+            update={
+                "status": ExportStatus.cancelled,
+                "completed_at": _now(),
+            }
+        )
+    except Exception as exc:  # 渲染失败不拖垮服务，只记日志与项目错误码
+        logger.exception("Export failed: job_id=%s", job_id)
+        _jobs[job_id] = _jobs[job_id].model_copy(
+            update={
+                "status": ExportStatus.failed,
+                "error": "Export render failed.",
+                "error_code": "EXPORT_RENDER_FAILED",
+                "completed_at": _now(),
+            }
+        )
+    finally:
+        _cancel_flags.pop(job_id, None)
+
+
+def list_exports(
+    status: ExportStatus | None = None,
+    format: ExportFormat | None = None,
+    limit: int = 50,
+    offset: int = 0,
+) -> tuple[list[ExportJob], int]:
+    jobs = list(_jobs.values())
+    if status is not None:
+        jobs = [j for j in jobs if j.status == status]
+    if format is not None:
+        jobs = [j for j in jobs if j.format == format]
+    jobs.sort(key=lambda j: j.created_at, reverse=True)
+    total = len(jobs)
+    return jobs[offset : offset + limit], total
+
+
+def get_export(job_id: str) -> ExportJob | None:
+    return _jobs.get(job_id)
+
+
+def cancel_export(job_id: str) -> ExportJob | None:
+    """取消导出：仅 queued/running 可取消，后台 Task 在让出边界标记 cancelled。"""
+    job = _jobs.get(job_id)
+    if job is None:
+        return None
+    if job.status in (ExportStatus.queued, ExportStatus.running):
+        _cancel_flags[job_id].set()
+    return job
+
+
+def get_export_file(job_id: str) -> Path:
+    """返回可下载产物的存储路径；未完成返回 404、过期返回 410。"""
+    job = _jobs.get(job_id)
+    if job is None:
+        raise ApiError(404, "EXPORT_JOB_NOT_FOUND", "export job not found", {"job_id": job_id})
+    if job.status != ExportStatus.completed or job.file is None:
+        raise ApiError(
+            404, "EXPORT_JOB_NOT_FOUND", "export file not ready", {"job_id": job_id}
+        )
+    if job.file.expires_at <= _now():
+        raise ApiError(410, "EXPORT_FILE_EXPIRED", "export file has expired", {"job_id": job_id})
+    return get_settings().exports_path / f"{job_id}.html"
+
+
+async def wait_for_export(job_id: str) -> ExportJob | None:
+    """等待后台任务结束（测试/轮询用）；无任务时直接返回当前状态。"""
+    task = _tasks.get(job_id)
+    if task is not None:
+        await task
+    return _jobs.get(job_id)