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/models/__init__.py

@@ -10,7 +10,7 @@ from app.domains.products.models import CadFile, Product
 from app.domains.orders.models import Order, OrderItem, OrderLine
 from app.domains.notifications.models import AuditLog
 from app.domains.billing.models import PricingTier
-from app.domains.rendering.models import OutputType, RenderTemplate, ProductRenderPosition
+from app.domains.rendering.models import OutputType, RenderTemplate, ProductRenderPosition, WorkflowDefinition, WorkflowRun, WorkflowNodeResult
 from app.domains.materials.models import Material, MaterialAlias
 
 # Also re-export SystemSetting (no domain assigned — stays as-is)
@@ -19,5 +19,6 @@ from app.models.system_setting import SystemSetting
 __all__ = [
     "Tenant", "User", "Template", "CadFile", "Product", "Order", "OrderItem", "OrderLine",
     "AuditLog", "PricingTier", "OutputType", "RenderTemplate", "ProductRenderPosition",
+    "WorkflowDefinition", "WorkflowRun", "WorkflowNodeResult",
     "Material", "MaterialAlias", "SystemSetting",
 ]