Add Flower for Celery monitoring

Flower provides a web UI to monitor:
- Active/completed/failed tasks
- Worker status and stats
- Task history and details

Accessible on port 5555.

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

docker-compose.yml

@@ -89,6 +89,22 @@ services:
       restart_policy:
         condition: on-failure
 
+  flower:
+    image: mher/flower:2.0
+    command: celery --broker=redis://redis:6379/5 flower --port=5555
+    environment:
+      - CELERY_BROKER_URL=redis://redis:6379/5
+      - FLOWER_PORT=5555
+    depends_on:
+      - redis
+    networks:
+      - celery
+      - externalnet
+    deploy:
+      replicas: 1
+      restart_policy:
+        condition: on-failure
+
 volumes:
   redis_data:
   postgres_data: