feat(phase8.1-8.2): dynamic worker concurrency via worker_configs

- Migration 054: worker_configs table (queue_name PK, max/min_concurrency,
  enabled, updated_at); seeds step_processing(8/2), thumbnail_rendering(1/1),
  ai_validation(4/1)
- WorkerConfig SQLAlchemy model
- apply_worker_concurrency beat task: reads enabled configs, broadcasts
  pool_grow to all Celery workers every 5min
- GET/PUT /api/worker/configs (admin): list + update per-queue concurrency
- docker-compose.yml: worker uses --autoscale=${MAX_CONCURRENCY:-8},${MIN_CONCURRENCY:-2};
  render-worker uses --autoscale=1,1 --concurrency=1
- WorkerManagement.tsx: "Concurrency Settings" section with +/- steppers
  and Save button per queue

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/tasks/celery_app.py

@@ -46,5 +46,9 @@ celery_app.conf.update(
             "task": "app.tasks.beat_tasks.recover_stalled_renders",
             "schedule": 300.0,  # every 5 minutes
         },
+        "apply-worker-concurrency-every-5m": {
+            "task": "app.tasks.beat_tasks.apply_worker_concurrency",
+            "schedule": 300.0,  # every 5 minutes
+        },
     },
 )