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/views/_shared.py

@@ -10,7 +10,8 @@ from __future__ import annotations
 
 # 分片上传参数（home / upload 共用）
 DEFAULT_CHUNK_SIZE = 4 * 1024 * 1024
-DEFAULT_CONCURRENCY = 3
+DEFAULT_CONCURRENCY = 3       # 单文件内分片并发数
+DEFAULT_FILE_CONCURRENCY = 5  # 同时上传的文件数（不阻塞 web 请求/ffmpeg/gpu）
 MAX_RETRY = 2
 POLL_INTERVAL = 2000
 
@@ -259,7 +260,8 @@ def render_upload_js(on_complete: str) -> str:
     """
     return f"""
 const CHUNK_SIZE = {DEFAULT_CHUNK_SIZE};
-const CONCURRENCY = {DEFAULT_CONCURRENCY};
+const CHUNK_CONCURRENCY = {DEFAULT_CONCURRENCY};   // 单文件内分片并发
+const FILE_CONCURRENCY = {DEFAULT_FILE_CONCURRENCY}; // 同时上传的文件数
 const MAX_RETRY = {MAX_RETRY};
 const UPLOAD_API = "/api/tasks/chunk-uploads";
 
@@ -315,13 +317,13 @@ function setUploadProgress(t, pct) {{
 }}
 
 function pump() {{
+  // 统计正在上传的文件数，启动等待中的文件直到达到 FILE_CONCURRENCY
   const active = pending.filter(t => t.state === "running").length;
   for (const t of pending) {{
-    if (active >= CONCURRENCY) break;
+    if (active >= FILE_CONCURRENCY) break;
     if (t.state === "pending") {{
-      t.state = "running";
+      setUploadState(t, "running");
       startUpload(t);
-      active++;
     }}
   }}
 }}
@@ -342,7 +344,7 @@ async function startUpload(t) {{
 
     const need = [];
     for (let i = 0; i < t.totalChunks; i++) if (!t.uploaded.has(i)) need.push(i);
-    await runPool(need, CONCURRENCY, i => uploadChunk(t, i));
+    await runPool(need, CHUNK_CONCURRENCY, i => uploadChunk(t, i));
     if (t.uploaded.size < t.totalChunks) throw new Error("部分分片未能上传");
 
     setUploadState(t, "hashing");
@@ -359,6 +361,9 @@ async function startUpload(t) {{
     meta.className = "task-meta fail-msg";
     meta.textContent = String(e.message || e);
     t.el.appendChild(meta);
+  }} finally {{
+    // 无论成功还是失败，都触发 pump 让队列中下一个文件开始上传
+    pump();
   }}
 }}