feat: workflow graph system — blocks 1-20 complete checkpoint

Full graph-based workflow execution engine with:
- WorkflowGraphRuntime with two-phase dispatch (collect → fire Celery tasks)
- Node registry with contract validation, socket types, execution kinds
- WorkflowRuntimeServices: preflight, context resolution, shadow-mode A/B
- WorkflowRouter: CRUD + dispatch/preflight/run-history API endpoints
- OutputTypeContracts: workflow binding, rollout-mode resolution
- Admin router: output-type workflow binding endpoints
- Frontend: complete workflow editor with drag-drop canvas, node inspector,
  module bundles, reference bundles, preflight panel, validation banner,
  authoring guidance, blueprint templates, shadow/rollout gate UI
- Tests: comprehensive coverage for all workflow modules
- Docs: NODE_CONTRACT_AUDIT and VALIDATION_ERROR_INVENTORY

All 20 blocks from NEXT_20_BLOCK_BATCH_PLAN completed.

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

backend/app/domains/rendering/workflow_node_registry.py

@@ -1058,10 +1058,10 @@ _NODE_DEFINITIONS: list[WorkflowNodeDefinition] = [
         input_contract={
             "context": "order_line",
             "requires": ["order_line_context"],
-            "requires_any": ["rendered_image", "rendered_frames", "rendered_video"],
+            "requires_any": ["rendered_image", "rendered_frames", "rendered_video", "blend_asset"],
         },
         output_contract={"context": "order_line", "provides": ["media_asset", "workflow_result"]},
-        artifact_roles_consumed=["rendered_image", "rendered_frames", "rendered_video"],
+        artifact_roles_consumed=["rendered_image", "rendered_frames", "rendered_video", "blend_asset"],
         artifact_roles_produced=["media_asset", "workflow_result"],
     ),
     _definition(
@@ -1140,10 +1140,16 @@ _NODE_DEFINITIONS: list[WorkflowNodeDefinition] = [
         input_contract={
             "context": "order_line",
             "requires": ["order_line_context"],
-            "requires_any": ["rendered_image", "rendered_frames", "rendered_video", "workflow_result"],
+            "requires_any": [
+                "rendered_image",
+                "rendered_frames",
+                "rendered_video",
+                "workflow_result",
+                "blend_asset",
+            ],
         },
         output_contract={"context": "order_line", "provides": ["notification_event"]},
-        artifact_roles_consumed=["workflow_result"],
+        artifact_roles_consumed=["workflow_result", "blend_asset"],
         artifact_roles_produced=["notification_event"],
     ),
 ]