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/domains/rendering/schemas.py

@@ -89,3 +89,51 @@ class RenderPositionOut(BaseModel):
     updated_at: datetime
 
     model_config = {"from_attributes": True}
+
+
+class WorkflowDefinitionCreate(BaseModel):
+    name: str
+    output_type_id: uuid.UUID | None = None
+    config: dict
+    is_active: bool = True
+
+
+class WorkflowDefinitionUpdate(BaseModel):
+    name: str | None = None
+    config: dict | None = None
+    is_active: bool | None = None
+
+
+class WorkflowDefinitionOut(BaseModel):
+    id: uuid.UUID
+    name: str
+    output_type_id: uuid.UUID | None
+    config: dict
+    is_active: bool
+    created_at: datetime
+    model_config = {"from_attributes": True}
+
+
+class WorkflowNodeResultOut(BaseModel):
+    id: uuid.UUID
+    node_name: str
+    status: str
+    output: dict | None
+    log: str | None
+    duration_s: float | None
+    created_at: datetime
+    model_config = {"from_attributes": True}
+
+
+class WorkflowRunOut(BaseModel):
+    id: uuid.UUID
+    workflow_def_id: uuid.UUID | None
+    order_line_id: uuid.UUID | None
+    celery_task_id: str | None
+    status: str
+    started_at: datetime | None
+    completed_at: datetime | None
+    error_message: str | None
+    created_at: datetime
+    node_results: list[WorkflowNodeResultOut] = []
+    model_config = {"from_attributes": True}