feat(C1+C2): workflow system — WorkflowDefinition + Celery Canvas builder

Migrations 037 (workflow tables + 3 seed definitions) + 038 (output_types.workflow_definition_id).
WorkflowDefinition/Run/NodeResult SQLAlchemy models in domains/rendering/models.py.
workflow_builder.py: dispatch_workflow() with Celery Canvas for still/turntable/multi_angle.
workflow_router.py: CRUD endpoints at /api/workflows (admin/PM guards).
dispatch_service.py: dispatch_render_with_workflow() prefers workflow path when
  OutputType.workflow_definition_id is set, falls back to legacy dispatch otherwise.
main.py: registers workflows_router.
models/__init__.py: re-exports WorkflowDefinition, WorkflowRun, WorkflowNodeResult.

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

backend/app/main.py

@@ -18,6 +18,7 @@ from app.domains.rendering.router import render_templates_router, output_types_r
 from app.domains.notifications.router import router as notifications_router
 from app.domains.billing.router import router as pricing_router
 from app.domains.tenants.router import router as tenants_router
+from app.domains.rendering.workflow_router import router as workflows_router
 
 
 @asynccontextmanager
@@ -76,6 +77,7 @@ app.include_router(output_types_router, prefix="/api")
 app.include_router(render_templates_router, prefix="/api")
 app.include_router(notifications_router, prefix="/api")
 app.include_router(tenants_router, prefix="/api")
+app.include_router(workflows_router)
 
 
 @app.get("/health")