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_schema.py

@@ -399,6 +399,54 @@ def test_workflow_schema_accepts_transitive_contract_wiring():
     assert config.ui.execution_mode == "graph"
 
 
+def test_workflow_schema_accepts_export_blend_notify_contract_wiring():
+    config = WorkflowConfig.model_validate(
+        {
+            "version": 1,
+            "nodes": [
+                {"id": "setup", "step": "order_line_setup", "params": {}},
+                {"id": "template", "step": "resolve_template", "params": {}},
+                {"id": "export", "step": "export_blend", "params": {"output_name_suffix": "review"}},
+                {"id": "notify", "step": "notify", "params": {}},
+            ],
+            "edges": [
+                {"from": "setup", "to": "template"},
+                {"from": "setup", "to": "export"},
+                {"from": "template", "to": "export"},
+                {"from": "export", "to": "notify"},
+            ],
+            "ui": {"family": "order_line", "execution_mode": "graph"},
+        }
+    )
+
+    assert config.ui is not None
+    assert config.ui.execution_mode == "graph"
+
+
+def test_workflow_schema_accepts_export_blend_output_save_contract_wiring():
+    config = WorkflowConfig.model_validate(
+        {
+            "version": 1,
+            "nodes": [
+                {"id": "setup", "step": "order_line_setup", "params": {}},
+                {"id": "template", "step": "resolve_template", "params": {}},
+                {"id": "export", "step": "export_blend", "params": {"output_name_suffix": "review"}},
+                {"id": "output", "step": "output_save", "params": {"expected_artifact_role": "blend_export"}},
+            ],
+            "edges": [
+                {"from": "setup", "to": "template"},
+                {"from": "setup", "to": "export"},
+                {"from": "template", "to": "export"},
+                {"from": "export", "to": "output"},
+            ],
+            "ui": {"family": "order_line", "execution_mode": "graph"},
+        }
+    )
+
+    assert config.ui is not None
+    assert config.ui.execution_mode == "graph"
+
+
 def test_workflow_schema_accepts_cad_intake_contract_wiring_with_shared_bbox_node():
     config = WorkflowConfig.model_validate(
         {