Fix Celery prefetching deadlock by enabling Fair Scheduling

start.sh

@@ -2,7 +2,7 @@
 
 # Start Celery worker in background
 # We use concurrency=2 to avoid memory overload on the 16GB free tier
-celery -A tasks worker --loglevel=info --concurrency=2 -B &
+celery -A tasks worker --loglevel=info --concurrency=2 -O fair -B &
 CELERY_PID=$!
 
 # Trap SIGTERM and SIGINT for graceful shutdown

tasks.py

@@ -76,6 +76,8 @@ celery_app.conf.update(
     task_time_limit=3600,  # 1 hour hard limit
     task_soft_time_limit=3300,  # 55 min soft limit
     result_expires=86400,  # 24 hours
+    worker_prefetch_multiplier=1,  # Prevent long-running tasks from getting stuck behind each other
+    task_acks_late=True,  # Only acknowledge task after it completely finishes
 )
 
 extra_conf: dict[str, Any] = {}