feat: add scheduled message polling for avatar takeover

Add APScheduler BackgroundScheduler to on_startup that polls
takeover_service.poll_and_process_messages every 10 seconds. Added
redis and apscheduler dependencies. Added poll_and_process_messages,
fetch_unread_messages, and process_message methods to TakeoverService.
Scheduler initialization is graceful — Redis/Box IM failures do not
prevent app startup.

digital-avatar-app/backend/services/takeover_service.py

@@ -144,3 +144,31 @@ class TakeoverService:
                 self.redis.delete(key)
         except Exception as e:
             logger.error(f"Failed to process delayed queue: {e}")
+
+    async def poll_and_process_messages(self):
+        """Periodic polling job: fetch unread messages and process each."""
+        try:
+            messages = await self.fetch_unread_messages()
+            for msg in messages:
+                await self.process_message(msg)
+        except Exception as e:
+            logger.error(f"poll_and_process_messages failed: {e}")
+
+    async def fetch_unread_messages(self) -> list:
+        """Fetch unread messages from Box IM. Stub — replace with real API call."""
+        logger.debug("fetch_unread_messages: no real API wired yet")
+        return []
+
+    async def process_message(self, message: dict):
+        """Process a single message: check takeover, dispatch immediate or delayed."""
+        owner_id = message.get("owner_huihui_id", "")
+        from_id = message.get("from_accid", "")
+
+        auth = self.check_takeover_enabled(owner_id, from_id)
+        if not auth:
+            return
+
+        if auth.takeover_mode == "immediate":
+            await self.execute_takeover(auth, message)
+        else:
+            self.enqueue_delayed_message(auth, message)