feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

backend/internal/service/billing_cache_service_test.go

@@ -70,7 +70,7 @@ func (b *billingCacheWorkerStub) InvalidateAPIKeyRateLimit(ctx context.Context,
 
 func TestBillingCacheServiceQueueHighLoad(t *testing.T) {
 	cache := &billingCacheWorkerStub{}
-	svc := NewBillingCacheService(cache, nil, nil, nil, &config.Config{})
+	svc := NewBillingCacheService(cache, nil, nil, nil, nil, nil, &config.Config{})
 	t.Cleanup(svc.Stop)
 
 	start := time.Now()
@@ -92,7 +92,7 @@ func TestBillingCacheServiceQueueHighLoad(t *testing.T) {
 
 func TestBillingCacheServiceEnqueueAfterStopReturnsFalse(t *testing.T) {
 	cache := &billingCacheWorkerStub{}
-	svc := NewBillingCacheService(cache, nil, nil, nil, &config.Config{})
+	svc := NewBillingCacheService(cache, nil, nil, nil, nil, nil, &config.Config{})
 	svc.Stop()
 
 	enqueued := svc.enqueueCacheWrite(cacheWriteTask{