fix: handle malformed JSON payloads in worker with error logging

backend/worker.py

@@ -49,9 +49,13 @@ async def process_random_number_job(job: Job, db_pool: asyncpg.Pool) -> None:
 
     # Parse payload
     payload_str = job.payload.decode() if job.payload else "{}"
-    payload = json.loads(payload_str)
-    user_id = payload.get("user_id")
+    try:
+        payload = json.loads(payload_str)
+    except json.JSONDecodeError as e:
+        logger.error(f"Job {job.id}: Invalid JSON payload: {e}")
+        return
 
+    user_id = payload.get("user_id")
     if user_id is None:
         logger.error(f"Job {job.id}: Missing user_id in payload")
         return