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/workflow_builder.py

@@ -0,0 +1,53 @@
+"""Celery Canvas workflow builder.
+
+Translates WorkflowDefinition config into a Celery Canvas (chain/group/chord).
+"""
+from __future__ import annotations
+import logging
+from celery import chain, group
+
+logger = logging.getLogger(__name__)
+
+
+def dispatch_workflow(
+    workflow_type: str,
+    order_line_id: str,
+    params: dict | None = None,
+) -> str:
+    """Build and dispatch a Celery Canvas workflow. Returns the Celery task/group ID."""
+    params = params or {}
+    builders = {
+        "still": _build_still,
+        "turntable": _build_turntable,
+        "multi_angle": _build_multi_angle,
+    }
+    builder = builders.get(workflow_type)
+    if not builder:
+        raise ValueError(f"Unknown workflow type: {workflow_type!r}")
+    canvas = builder(order_line_id, params)
+    result = canvas.apply_async()
+    return str(result.id)
+
+
+def _build_still(order_line_id: str, params: dict):
+    from app.domains.rendering.tasks import render_still_task
+    return chain(
+        render_still_task.si(order_line_id, **params)
+    )
+
+
+def _build_turntable(order_line_id: str, params: dict):
+    from app.domains.rendering.tasks import render_turntable_task
+    return chain(
+        render_turntable_task.si(order_line_id, **params)
+    )
+
+
+def _build_multi_angle(order_line_id: str, params: dict):
+    from app.domains.rendering.tasks import render_still_task
+    angles = params.get("angles", [0, 45, 90])
+    p = {k: v for k, v in params.items() if k != "angles"}
+    return group(
+        render_still_task.si(order_line_id, camera_angle=angle, **p)
+        for angle in angles
+    )