refactor: 重构 schemas 结构，统一枚举定义

backend/tests/unit/v1/inbox_messages/test_schemas.py

@@ -16,7 +16,7 @@ def test_inbox_message_response_schema() -> None:
         sender_id=uuid4(),
         message_type=InboxMessageType.CALENDAR,
         schedule_item_id=uuid4(),
-        content="Join my calendar",
+        content={"type": "invite", "permission": 1, "action": "pending"},
         is_read=False,
         status=InboxMessageStatus.PENDING,
         created_at=datetime.now(timezone.utc),

backend/tests/unit/v1/inbox_messages/test_service.py

@@ -22,7 +22,7 @@ def _build_message(
     status: InboxMessageModelStatus = InboxMessageModelStatus.PENDING,
     message_type: InboxMessageModelType = InboxMessageModelType.CALENDAR,
     schedule_item_id: UUID | None = None,
-    content: str = '{"permission": 7}',
+    content: dict[str, object] = {"permission": 7},
 ) -> InboxMessage:
     message = MagicMock(spec=InboxMessage)
     message.id = message_id