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>

frontend/src/components/workflows/workflowAuthoringGuidance.ts

@@ -103,8 +103,8 @@ function getAuthoringPriorities(graphFamily: WorkflowGraphFamily): WorkflowAutho
         description: 'Insert the full non-legacy still production baseline first, then tune modules or individual nodes.',
       },
       {
-        title: 'Swap stages with production modules',
-        description: 'Use render and publish bundles to change whole stages without breaking graph-safe sequencing.',
+        title: 'Swap stages with stage modules',
+        description: 'Use scene-prep, materials, render, and publish bundles to change whole production stages without breaking graph-safe sequencing.',
       },
       {
         title: 'Use raw nodes last',
@@ -160,7 +160,7 @@ export function getWorkflowAuthoringFlow(graphFamily: WorkflowGraphFamily): Work
     },
     {
       index: 2,
-      title: 'Production Modules',
+      title: 'Stage Modules',
       description: 'Swap or extend whole production stages with reusable graph-safe bundles.',
     },
     {
@@ -186,7 +186,7 @@ export function getWorkflowAuthoringPlan(
   const title = graphFamily === 'mixed' ? 'Guided Authoring' : STARTER_PATH_TITLES[graphFamily]
   const description =
     graphFamily === 'mixed'
-      ? 'Keep the graph readable by preferring complete paths and bundles before raw node-level edits.'
+      ? 'Keep the graph readable by preferring complete paths and stage bundles before raw node-level edits.'
       : STARTER_PATH_DESCRIPTIONS[graphFamily]
   const referenceBundles = getWorkflowReferenceBundles(definitions, graphFamily)
     .map(bundle => ({