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/accounts.py

@@ -11,6 +11,7 @@ from ..schemas import AccountImport, AccountAssign, AccountTag, AccountOut, Batc
 from ..deps import get_current_user, require_permission
 from ..permissions import has_permission
 from sqlalchemy import func
+from sqlalchemy.orm import joinedload
 
 router = APIRouter(prefix="/api/accounts", tags=["账号管理"])
 
@@ -64,7 +65,7 @@ def list_accounts(
     if tag:
         query = query.filter(Account.tag == tag)
 
-    accounts = query.order_by(Account.id).all()
+    accounts = query.order_by(Account.id).options(joinedload(Account.assigned_user)).all()
     result = []
     for acc in accounts:
         item = AccountOut(