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/models/__init__.py

@@ -17,11 +17,12 @@ from app.domains.admin.models import DashboardConfig
 
 # Also re-export SystemSetting (no domain assigned — stays as-is)
 from app.models.system_setting import SystemSetting
+from app.models.worker_config import WorkerConfig
 
 __all__ = [
     "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
     "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
     "WorkflowDefinition", "WorkflowRun", "WorkflowNodeResult",
     "Material", "MaterialAlias", "AssetLibrary", "MediaAsset", "MediaAssetType", "SystemSetting",
-    "DashboardConfig",
+    "DashboardConfig", "WorkerConfig",
 ]