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/cmd/server/wire_gen_test.go

@@ -43,7 +43,7 @@ func TestProvideCleanup_WithMinimalDependencies_NoPanic(t *testing.T) {
 	subscriptionExpirySvc := service.NewSubscriptionExpiryService(nil, time.Second)
 	pricingSvc := service.NewPricingService(cfg, nil)
 	emailQueueSvc := service.NewEmailQueueService(nil, 1)
-	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, nil, cfg)
+	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, nil, nil, nil, cfg)
 	idempotencyCleanupSvc := service.NewIdempotencyCleanupService(nil, cfg)
 	schedulerSnapshotSvc := service.NewSchedulerSnapshotService(nil, nil, nil, nil, cfg)
 	opsSystemLogSinkSvc := service.NewOpsSystemLogSink(nil)