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/wire.go

@@ -39,6 +39,11 @@ func ProvideEmailQueueService(emailService *EmailService) *EmailQueueService {
 	return NewEmailQueueService(emailService, 3)
 }
 
+// ProvideOAuthRefreshAPI creates OAuthRefreshAPI with the default lock TTL.
+func ProvideOAuthRefreshAPI(accountRepo AccountRepository, tokenCache GeminiTokenCache) *OAuthRefreshAPI {
+	return NewOAuthRefreshAPI(accountRepo, tokenCache)
+}
+
 // ProvideTokenRefreshService creates and starts TokenRefreshService
 func ProvideTokenRefreshService(
 	accountRepo AccountRepository,
@@ -383,6 +388,19 @@ func ProvideSettingService(settingRepo SettingRepository, groupRepo GroupReposit
 	return svc
 }
 
+// ProvideBillingCacheService wires BillingCacheService with its RPM dependencies.
+func ProvideBillingCacheService(
+	cache BillingCache,
+	userRepo UserRepository,
+	subRepo UserSubscriptionRepository,
+	apiKeyRepo APIKeyRepository,
+	rpmCache UserRPMCache,
+	rateRepo UserGroupRateRepository,
+	cfg *config.Config,
+) *BillingCacheService {
+	return NewBillingCacheService(cache, userRepo, subRepo, apiKeyRepo, rpmCache, rateRepo, cfg)
+}
+
 // ProviderSet is the Wire provider set for all services
 var ProviderSet = wire.NewSet(
 	// Core services
@@ -399,7 +417,7 @@ var ProviderSet = wire.NewSet(
 	NewDashboardService,
 	ProvidePricingService,
 	NewBillingService,
-	NewBillingCacheService,
+	ProvideBillingCacheService,
 	NewAnnouncementService,
 	NewAdminService,
 	NewGatewayService,
@@ -411,7 +429,7 @@ var ProviderSet = wire.NewSet(
 	NewCompositeTokenCacheInvalidator,
 	wire.Bind(new(TokenCacheInvalidator), new(*CompositeTokenCacheInvalidator)),
 	NewAntigravityOAuthService,
-	NewOAuthRefreshAPI,
+	ProvideOAuthRefreshAPI,
 	ProvideGeminiTokenProvider,
 	NewGeminiMessagesCompatService,
 	ProvideAntigravityTokenProvider,