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/api/routers/admin.py

@@ -735,6 +735,14 @@ async def seed_workflows(
             "name": "Still Graph Blueprint",
             "config": build_workflow_blueprint_config("still_graph_reference"),
         },
+        {
+            "name": "Still Graph Alpha Blueprint",
+            "config": build_workflow_blueprint_config("still_graph_alpha_reference"),
+        },
+        {
+            "name": "Still Graph Blend Blueprint",
+            "config": build_workflow_blueprint_config("still_graph_blend_reference"),
+        },
     ]
 
     existing_result = await db.execute(select(WorkflowDefinition))