fix: Celery 异步任务 event loop 冲突修复

- 原因: 全局 async_session_factory 绑定在 Celery 主进程 event loop
- Celery fork worker 后创建新 loop，旧 connection 绑定冲突
- 修复: 每个任务内创建独立 async engine + session factory
- 任务完成后 dispose engine 释放连接

backend/app/tasks/generate.py

@@ -1,9 +1,8 @@
 import os
-import uuid
 from datetime import datetime, timezone
 from sqlalchemy import select
+from sqlalchemy.ext.asyncio import create_async_engine, async_sessionmaker, AsyncSession
 from app.tasks.celery_app import celery_app
-from app.core.database import async_session_factory
 from app.core.config import get_settings
 from app.models.generation_task import GenerationTask
 from app.models.generation_point import GenerationPoint
@@ -14,10 +13,22 @@ from app.services.ref_parser import parse_reference_file
 from app.services.file_storage import get_storage_dir, RESULTS_DIR
 
 
-async def _generate_document(task_id: str) -> None:
+def _create_db_session() -> async_sessionmaker[AsyncSession]:
     settings = get_settings()
+    engine = create_async_engine(
+        settings.DATABASE_URL,
+        echo=False,
+        pool_size=5,
+        max_overflow=5,
+        pool_pre_ping=True,
+    )
+    return async_sessionmaker(engine, class_=AsyncSession, expire_on_commit=False)
 
-    async with async_session_factory() as db:
+
+async def _generate_document(task_id: str) -> None:
+    session_factory = _create_db_session()
+
+    async with session_factory() as db:
         task_result = await db.execute(select(GenerationTask).where(GenerationTask.id == task_id))
         task = task_result.scalar_one_or_none()
         if not task:
@@ -45,8 +56,7 @@ async def _generate_document(task_id: str) -> None:
         points = points_result.scalars().all()
 
         try:
-            total = len(points)
+            for point in points:
-            for idx, point in enumerate(points):
                 ref_content = None
                 if point.ref_file_path:
                     ref_content = await parse_reference_file(point.ref_file_path)
@@ -72,7 +82,6 @@ async def _generate_document(task_id: str) -> None:
                 if 0 <= start < end <= len(html_content):
                     html_content = html_content[:start] + ai_result + html_content[end:]
 
-                task.status = "processing"
                 await db.commit()
 
             result_dir = get_storage_dir(RESULTS_DIR)
@@ -92,6 +101,8 @@ async def _generate_document(task_id: str) -> None:
             task.finished_at = datetime.now(timezone.utc)
             await db.commit()
 
+    await session_factory.engine.dispose()
+
 
 @celery_app.task(bind=True, name="generate_document")
 def generate_document(self, task_id: str) -> dict: