feat(phase8.1-8.2): dynamic worker concurrency via worker_configs

- Migration 054: worker_configs table (queue_name PK, max/min_concurrency,
  enabled, updated_at); seeds step_processing(8/2), thumbnail_rendering(1/1),
  ai_validation(4/1)
- WorkerConfig SQLAlchemy model
- apply_worker_concurrency beat task: reads enabled configs, broadcasts
  pool_grow to all Celery workers every 5min
- GET/PUT /api/worker/configs (admin): list + update per-queue concurrency
- docker-compose.yml: worker uses --autoscale=${MAX_CONCURRENCY:-8},${MIN_CONCURRENCY:-2};
  render-worker uses --autoscale=1,1 --concurrency=1
- WorkerManagement.tsx: "Concurrency Settings" section with +/- steppers
  and Save button per queue

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

frontend/src/api/worker.ts

@@ -166,3 +166,34 @@ export async function scaleWorkers(req: ScaleRequest): Promise<ScaleResponse> {
   const res = await api.post<ScaleResponse>('/worker/scale', req)
   return res.data
 }
+
+// ---------------------------------------------------------------------------
+// Worker concurrency configuration
+// ---------------------------------------------------------------------------
+
+export interface WorkerConfig {
+  queue_name: string
+  max_concurrency: number
+  min_concurrency: number
+  enabled: boolean
+  updated_at: string
+}
+
+export interface WorkerConfigUpdate {
+  max_concurrency?: number
+  min_concurrency?: number
+  enabled?: boolean
+}
+
+export async function getWorkerConfigs(): Promise<WorkerConfig[]> {
+  const res = await api.get<WorkerConfig[]>('/worker/configs')
+  return res.data
+}
+
+export async function updateWorkerConfig(
+  queueName: string,
+  update: WorkerConfigUpdate,
+): Promise<WorkerConfig> {
+  const res = await api.put<WorkerConfig>(`/worker/configs/${queueName}`, update)
+  return res.data
+}