fix(backend): 修复全面审阅发现的核心问题

修复索引首次失败回滚、Vault 扫描边界、Markdown 代码围栏和 UTF-16 Citation 偏移。

收紧 Plugin 权限与 JSON Schema 校验，补齐 Note Move、Task、Attachment 和 Transcript Tool。

接入 OpenAI SSE 与 Ollama JSONL 真流式输出，修正 Provider PATCH 语义并限制运行时内存保留。

新增对应回归测试，后端测试增至 62 项。

backend/app/services/attachment_service.py

@@ -0,0 +1,46 @@
+from __future__ import annotations
+
+import re
+from pathlib import Path
+
+from app.config import get_settings
+from app.errors import ApiError
+
+_ATTACHMENT_ID = re.compile(r"^[A-Za-z0-9][A-Za-z0-9._-]{0,254}$")
+MAX_ATTACHMENT_BYTES = 1024 * 1024
+
+
+def attachment_path(attachment_id: str) -> Path:
+    if not _ATTACHMENT_ID.fullmatch(attachment_id):
+        raise ApiError(
+            400, "INVALID_ATTACHMENT_ID", "attachment_id is invalid",
+            {"attachment_id": attachment_id},
+        )
+    root = get_settings().attachments_path.resolve()
+    candidate = (root / attachment_id).resolve()
+    if not candidate.is_relative_to(root):
+        raise ApiError(400, "INVALID_ATTACHMENT_ID", "attachment path escapes storage")
+    return candidate
+
+
+def read_attachment(attachment_id: str, *, max_chars: int = 100_000) -> dict[str, object]:
+    path = attachment_path(attachment_id)
+    if not path.is_file():
+        raise ApiError(
+            404, "ATTACHMENT_NOT_FOUND", "attachment not found",
+            {"attachment_id": attachment_id},
+        )
+    size = path.stat().st_size
+    if size > MAX_ATTACHMENT_BYTES:
+        raise ApiError(
+            413, "ATTACHMENT_TOO_LARGE", "attachment exceeds the Tool read limit",
+            {"attachment_id": attachment_id, "size": size},
+        )
+    text = path.read_text(encoding="utf-8")
+    truncated = len(text) > max_chars
+    return {
+        "attachment_id": attachment_id,
+        "content": text[:max_chars],
+        "size": size,
+        "truncated": truncated,
+    }