fix: add render queue to Celery worker

The accessible video render task was being dispatched to the 'render' queue
but no worker was listening to it. Added 'render' to:
- Dockerfile CMD args for worker queue list
- celery_worker.py import and log message

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/Dockerfile

@@ -134,7 +134,7 @@ ENTRYPOINT ["tini", "--"]
 # Start Celery worker listening to all queues
 # --concurrency=4 for 4 worker processes (adjust based on CPU cores available)
 CMD ["celery", "-A", "celery_worker", "worker", \
-     "-Q", "default,ingest,notify", \
+     "-Q", "default,ingest,notify,render", \
      "--loglevel=info", \
      "--concurrency=4", \
      "--max-tasks-per-child=100"]

backend/celery_worker.py

@@ -24,12 +24,13 @@ logger.info("Starting Celery worker with structured logging")
 # Import task modules to register them
 from app.tasks import ingest_and_ai
 from app.tasks import translate_and_synthesize
+from app.tasks import render_accessible_video
 
 # Debug: Show registered tasks
 logger.info(f"Celery app: {celery_app}")
 logger.info(f"Registered tasks: {list(celery_app.tasks.keys())}")
 logger.info(f"Task routes: {celery_app.conf.task_routes}")
-logger.info(f"Worker listening to queues: default,ingest")
+logger.info(f"Worker listening to queues: default,ingest,notify,render")
 
 # Specifically check for our translation task
 if 'app.tasks.translate_and_synthesize.translate_and_synthesize_task' in celery_app.tasks: