refactor(api): staff-chat-stats 确定性排序 + last_is_send=None 分支测试 + 类型标注

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

yunke-server/app/api/routers/staff_chat_stats.py

@@ -36,7 +36,7 @@ def _private_window(msg, owner: str, start: int, end: int):
     ]
 
 
-async def _aggregate(session: AsyncSession, owner: str, start: int, end: int):
+async def _aggregate(session: AsyncSession, owner: str, start: int, end: int) -> tuple[list, dict[str, bool | None]]:
     """返回 [(talker_id, send, recv, interaction_days, last_time)] + last_is_send map。"""
     # 跨库日期提取：从 ms-epoch 取日期。镜像 repos.py 方言检测。
     dialect = session.bind.name  # 镜像 app/storage/repos.py（.name 等价 .dialect.name）
@@ -56,6 +56,7 @@ async def _aggregate(session: AsyncSession, owner: str, start: int, end: int):
         )
         .where(*base)
         .group_by(Message.talker_id)
+        .order_by(Message.talker_id)
     )
     agg_rows = (await session.execute(agg_stmt)).all()