修复登录任务页面加载慢的问题

- 后端: list_tasks 用 IN 批量查 Account 替代逐条查询，消除 N+1
- 后端: list_accounts 用 joinedload 预加载 assigned_user，消除懒加载
- 前端: 首次加载 accounts 和 tasks 并行请求

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/backend/routers/login.py

@@ -87,24 +87,28 @@ def list_tasks(
     current: User = Depends(get_current_user),
 ):
     """查看登录任务列表。"""
-    query = db.query(LoginTask)
+    query = db.query(LoginTask).join(Account, LoginTask.account_id == Account.id)
 
     # 客服只能看自己账号的任务
     if not has_permission(current.role, "login:view_all"):
-        query = query.join(Account, LoginTask.account_id == Account.id).filter(
-            Account.assigned_to == current.id
-        )
+        query = query.filter(Account.assigned_to == current.id)
 
     if batch_id:
         query = query.filter(LoginTask.batch_id == batch_id)
 
-    tasks = query.order_by(LoginTask.id.desc()).limit(200).all()
+    rows = query.order_by(LoginTask.id.desc()).limit(200).all()
+    # 批量收集 account_id，一次性查出 username
+    account_ids = [t.account_id for t in rows]
+    accounts_map = {}
+    if account_ids:
+        accs = db.query(Account).filter(Account.id.in_(account_ids)).all()
+        accounts_map = {a.id: a.username for a in accs}
+
     result = []
-    for t in tasks:
-        acc = db.query(Account).filter(Account.id == t.account_id).first()
+    for t in rows:
         result.append(LoginTaskOut(
             id=t.id, batch_id=t.batch_id, account_id=t.account_id,
-            account_username=acc.username if acc else "",
+            account_username=accounts_map.get(t.account_id, ""),
             status=t.status, cookie=t.cookie or "", message=t.message or "",
             created_by=t.created_by, created_at=t.created_at, finished_at=t.finished_at,
         ))