fix: 端点层operator_id兜底current_user.username — 管理员代办检测生效

根因: 移动端不传?operator_id=参数, 后端收到None
      _admin_proxy_note(None, assignee) → 不触发标记

修复: 6个端点(receive/reject/transfer/end/recall/spawn)
      operator_id or current_user.get(username) 兜底

backend/app/api/v1/endpoints/tasks.py

@@ -125,7 +125,8 @@ async def end_task_endpoint(
     用于工人认为工序已完结、无需转交下一人的场景。
     """
     return await task_service.end_task(
-        db, uuid.UUID(task_id), operator_id,
+        db, uuid.UUID(task_id),
+        operator_id or current_user.get("username", "") or None,
         operator_role=current_user.get("role"),
     )
 
@@ -146,7 +147,8 @@ async def recall_task_endpoint(
     适用场景：转交后发现选错人，在对方接收前撤回。
     """
     return await task_service.recall_task(
-        db, uuid.UUID(task_id), operator_id,
+        db, uuid.UUID(task_id),
+        operator_id or current_user.get("username", "") or None,
         operator_role=current_user.get("role"),
     )
 
@@ -173,7 +175,9 @@ async def spawn_subtask_endpoint(
     **派发协助分支：在当前任务下创建并行子任务，父任务状态保持不变。**
     用于 WIP 期间工人需要其他人协助协同的场景。
     """
-    return await task_service.spawn_subtask(db, uuid.UUID(task_id), data, operator_id)
+    return await task_service.spawn_subtask(
+        db, uuid.UUID(task_id), data,
+        operator_id or current_user.get("username", "") or None)
 
 
 # ============================================================
@@ -196,7 +200,9 @@ async def receive_task_endpoint(
     动作：状态改为 WIP，记录 received_at，更新 task_name，同步产品宏观状态。
     """
     return await task_service.receive_task(
-        db, uuid.UUID(task_id), operator_id, remark, task_name,
+        db, uuid.UUID(task_id),
+        operator_id or current_user.get("username", "") or None,
+        remark, task_name,
         operator_role=current_user.get("role"),
     )
 
@@ -222,7 +228,8 @@ async def reject_task_endpoint(
     3. 为该负责人新建返工任务（is_rework=True, status=PENDING）。
     """
     return await task_service.reject_task(
-        db, uuid.UUID(task_id), request, operator_id,
+        db, uuid.UUID(task_id), request,
+        operator_id or current_user.get("username", "") or None,
         operator_role=current_user.get("role"),
     )
 
@@ -256,7 +263,8 @@ async def transfer_task_endpoint(
     - 否则 → 顶层同级转交。
     """
     return await task_service.transfer_task(
-        db, uuid.UUID(task_id), request, operator_id,
+        db, uuid.UUID(task_id), request,
+        operator_id or current_user.get("username", "") or None,
         operator_role=current_user.get("role"),
     )