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/uv.lock

@@ -364,6 +364,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/41/45/1a4ed80516f02155c51f51e8cedb3c1902296743db0bbc66608a0db2814f/jsonschema_specifications-2025.9.1-py3-none-any.whl", hash = "sha256:98802fee3a11ee76ecaca44429fda8a41bff98b00a0f2838151b113f210cc6fe", size = 18437, upload-time = "2025-09-08T01:34:57.871Z" },
 ]
 
+[[package]]
+name = "mistune"
+version = "3.3.4"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/7b/92/328a294a6de83bacb95bed01f04e0eaff4e3616ee359fc821a5dfc539b02/mistune-3.3.4.tar.gz", hash = "sha256:58b5c96d6fcb61190dfe5fae498d2b2065f99cf61e9649418fd54cf1ada86dfe", size = 121426, upload-time = "2026-07-22T05:22:30.89Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/77/e4/288365afae98953bc01de09f686f40d8ee84578135aa7767d5d4e60b5278/mistune-3.3.4-py3-none-any.whl", hash = "sha256:ee015381e955e370962968befe1d729ab60fafb6a715ac6751763fbce38c8d4a", size = 66862, upload-time = "2026-07-22T05:22:29.419Z" },
+]
+
 [[package]]
 name = "notes-agent-backend"
 version = "0.1.0"
@@ -373,6 +382,7 @@ dependencies = [
     { name = "fastapi" },
     { name = "httpx" },
     { name = "jsonschema" },
+    { name = "mistune" },
     { name = "pyyaml" },
     { name = "referencing" },
     { name = "sqlite-vec" },
@@ -390,6 +400,7 @@ requires-dist = [
     { name = "fastapi", specifier = ">=0.116,<1.0" },
     { name = "httpx", specifier = ">=0.28,<1.0" },
     { name = "jsonschema", specifier = ">=4.25,<5.0" },
+    { name = "mistune", specifier = ">=3.0,<4.0" },
     { name = "pyyaml", specifier = ">=6.0,<7.0" },
     { name = "referencing", specifier = ">=0.36,<1.0" },
     { name = "sqlite-vec", specifier = ">=0.1.9" },