fix: 并发上传5文件刷新后丢失 + 分片写入阻塞事件循环

根因：
1. pump() 仅在 addFiles() 调用一次，上传完成/失败后不重新触发，
   导致 CONCURRENCY=3 之后的文件（4、5）永远不启动 → 无 DB 会话 → 刷新后消失
2. write_chunk 的 uploaded_chunks 是 read-modify-write，并发分片写入
   后者覆盖前者 → 分片记录丢失 → complete 报 409
3. upload_chunk 是 async 但同步调 write_chunk（fsync+DB commit），
   阻塞 uvicorn 事件循环 → 所有 web 请求被串行化

修复：
- _shared.py: pump() 加 finally 块，上传完成/失败后都触发下一文件；
  文件并发(FILE_CONCURRENCY=5)与分片并发(CHUNK_CONCURRENCY=3)分离
- upload_service.py: 按 upload_id 的进程级锁串行化 uploaded_chunks 更新，
  持锁后 db.refresh 重读最新值再 append，杜绝丢失更新；complete 后清理锁
- upload_router.py: upload_chunk 的 write_chunk 调用改用 run_in_threadpool，
  阻塞 I/O 移出事件循环，web 请求不再被分片写入阻塞

验证：5 文件并发上传后刷新全部可见；4 任务并发处理 4/4 成功(19.2s)

app/services/upload_service.py

@@ -16,6 +16,7 @@ from __future__ import annotations
 import logging
 import os
 import shutil
+import threading
 import uuid
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
@@ -36,6 +37,20 @@ from ..schemas.task import (
 
 logger = logging.getLogger("audio2text.upload")
 
+# 按 upload_id 串行化 uploaded_chunks 的读-改-写，避免并发分片写入丢失更新。
+# SQLite 无行锁，JSON 列的 append 操作不是原子的，必须进程内加锁。
+_chunk_locks: dict[str, threading.Lock] = {}
+_chunk_locks_guard = threading.Lock()
+
+
+def _get_chunk_lock(upload_id: str) -> threading.Lock:
+    with _chunk_locks_guard:
+        lock = _chunk_locks.get(upload_id)
+        if lock is None:
+            lock = threading.Lock()
+            _chunk_locks[upload_id] = lock
+        return lock
+
 
 class UploadService:
     def __init__(self, db: Session) -> None:
@@ -93,6 +108,7 @@ class UploadService:
         session_dir.mkdir(parents=True, exist_ok=True)
         chunk_path = session_dir / f"{index}.part"
 
+        # 先落盘（无锁，IO 可并行），再持锁更新 DB 计数
         try:
             with chunk_path.open("wb") as out:
                 out.write(data)
@@ -102,12 +118,16 @@ class UploadService:
             chunk_path.unlink(missing_ok=True)
             raise
 
-        uploaded = list(session.uploaded_chunks or [])
-        if index not in uploaded:
-            uploaded.append(index)
-        session.uploaded_chunks = uploaded
-        session.updated_at = datetime.now(timezone.utc)
-        self.db.commit()
+        # 持锁重读+更新：避免并发分片各自 read old list → append → write，后者覆盖前者
+        lock = _get_chunk_lock(upload_id)
+        with lock:
+            self.db.refresh(session)  # 拿最新 uploaded_chunks，不被其他并发请求的旧值覆盖
+            uploaded = list(session.uploaded_chunks or [])
+            if index not in uploaded:
+                uploaded.append(index)
+            session.uploaded_chunks = uploaded
+            session.updated_at = datetime.now(timezone.utc)
+            self.db.commit()
         return sorted(uploaded)
 
     # ---------------- 拼接 + 创建任务 ----------------
@@ -163,6 +183,9 @@ class UploadService:
 
         # 清理分片暂存（commit 后，即使清理失败也不影响已建任务）
         self._cleanup_session_dir(upload_id)
+        # 清理进程内锁，避免长期运行后 _chunk_locks 无限增长
+        with _chunk_locks_guard:
+            _chunk_locks.pop(upload_id, None)
 
         logger.info("上传完成 task_id=%s file=%s size=%d", task.id, session.filename, session.size_bytes)
         return CompleteResponse(