fix: workflow editor Phase 5/6 sign-off — save guard, conflict detection, dispatch pre-check

- Save button disabled for non-admins (canSave prop threaded through
  WorkflowEditor → WorkflowCanvas → WorkflowCanvasToolbar); tooltip
  explains why when hovered
- Optimistic concurrency: migration 072 adds updated_at to
  workflow_definitions; PUT /workflows/:id returns 409 when client sends
  a stale updated_at; frontend shows a specific reload-prompt toast
- _legacy_dispatch now routes through dispatch_order_line_render instead
  of calling render_order_line_task directly, so cancelled/rejected
  order lines are skipped before queueing

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

backend/app/domains/rendering/schemas.py

@@ -193,6 +193,7 @@ class WorkflowDefinitionUpdate(BaseModel):
     name: str | None = None
     config: dict | None = None
     is_active: bool | None = None
+    updated_at: datetime | None = None
 
 
 class WorkflowDefinitionOut(BaseModel):
@@ -207,6 +208,7 @@ class WorkflowDefinitionOut(BaseModel):
     )
     is_active: bool
     created_at: datetime
+    updated_at: datetime
     model_config = {"from_attributes": True}