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/tests/domains/test_workflow_executor.py

@@ -102,7 +102,7 @@ def test_dispatch_workflow_preserves_mapped_task_order_around_phase_3_bridge_nod
                 {"id": "setup", "step": StepName.ORDER_LINE_SETUP.value, "params": {}},
                 {"id": "render", "step": StepName.BLENDER_STILL.value, "params": {"width": 1024}},
                 {"id": "save", "step": StepName.OUTPUT_SAVE.value, "params": {}},
-                {"id": "export", "step": StepName.EXPORT_BLEND.value, "params": {"include_textures": True}},
+                {"id": "export", "step": StepName.EXPORT_BLEND.value, "params": {"output_name_suffix": "client"}},
                 {"id": "notify", "step": StepName.NOTIFY.value, "params": {}},
             ],
             "edges": [
@@ -125,6 +125,6 @@ def test_dispatch_workflow_preserves_mapped_task_order_around_phase_3_bridge_nod
         (
             "app.domains.rendering.tasks.export_blend_for_order_line_task",
             ["line-456"],
-            {"include_textures": True},
+            {"output_name_suffix": "client"},
         ),
     ]