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/models.py

@@ -166,6 +166,7 @@ class WorkflowDefinition(Base):
     config: Mapped[dict] = mapped_column(JSONB, nullable=False, default=dict)
     is_active: Mapped[bool] = mapped_column(Boolean, nullable=False, default=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
 
     runs: Mapped[list["WorkflowRun"]] = relationship(
         "WorkflowRun", back_populates="workflow_def", lazy="noload", cascade="all, delete-orphan"