feat(C3): add React Flow workflow editor

frontend/src/pages/WorkflowEditor.tsx: full React Flow editor with
custom nodes (Input/Convert/Render/FFmpeg/Output), config sidepanel,
node palette with drag-drop, new workflow dialog.
frontend/src/api/workflows.ts: workflow CRUD API client.
@xyflow/react added to package.json dependencies.
Route /workflows + sidebar link for admin+pm.

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

frontend/src/api/workflows.ts

@@ -0,0 +1,80 @@
+import api from './client'
+
+export interface WorkflowDefinition {
+  id: string
+  name: string
+  output_type_id: string | null
+  config: WorkflowConfig
+  is_active: boolean
+  created_at: string
+}
+
+export interface WorkflowConfig {
+  type: 'still' | 'turntable' | 'multi_angle' | 'custom'
+  params: WorkflowParams
+  nodes?: WorkflowNode[]
+}
+
+export interface WorkflowParams {
+  render_engine?: 'cycles' | 'eevee'
+  samples?: number
+  resolution?: [number, number]
+  fps?: number
+  duration_s?: number
+  angles?: number[]
+}
+
+export interface WorkflowNode {
+  id: string
+  type: string
+  position: { x: number; y: number }
+  data: Record<string, unknown>
+}
+
+export interface WorkflowCreate {
+  name: string
+  output_type_id?: string | null
+  config: WorkflowConfig
+  is_active?: boolean
+}
+
+export interface WorkflowRun {
+  id: string
+  workflow_def_id: string | null
+  order_line_id: string | null
+  celery_task_id: string | null
+  status: 'pending' | 'running' | 'completed' | 'failed'
+  started_at: string | null
+  completed_at: string | null
+  error_message: string | null
+  created_at: string
+  node_results: WorkflowNodeResult[]
+}
+
+export interface WorkflowNodeResult {
+  id: string
+  node_name: string
+  status: string
+  output: Record<string, unknown> | null
+  log: string | null
+  duration_s: number | null
+  created_at: string
+}
+
+export const getWorkflows = (): Promise<WorkflowDefinition[]> =>
+  api.get('/workflows').then(r => r.data)
+
+export const getWorkflow = (id: string): Promise<WorkflowDefinition> =>
+  api.get(`/workflows/${id}`).then(r => r.data)
+
+export const createWorkflow = (data: WorkflowCreate): Promise<WorkflowDefinition> =>
+  api.post('/workflows', data).then(r => r.data)
+
+export const updateWorkflow = (id: string, data: Partial<WorkflowCreate>): Promise<WorkflowDefinition> =>
+  api.put(`/workflows/${id}`, data).then(r => r.data)
+
+export const deleteWorkflow = (id: string): Promise<void> =>
+  api.delete(`/workflows/${id}`).then(() => undefined)
+
+export const getWorkflowRuns = (workflowId: string): Promise<WorkflowRun[]> =>
+  api.get(`/workflows/${workflowId}/runs`).then(r => r.data)