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/handler/gateway_handler_warmup_intercept_unit_test.go

@@ -173,7 +173,7 @@ func newTestGatewayHandler(t *testing.T, group *service.Group, accounts []*servi
 
 	// RunModeSimple：跳过计费检查，避免引入 repo/cache 依赖。
 	cfg := &config.Config{RunMode: config.RunModeSimple}
-	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, nil, cfg)
+	billingCacheSvc := service.NewBillingCacheService(nil, nil, nil, nil, nil, nil, cfg)
 
 	concurrencySvc := service.NewConcurrencyService(&fakeConcurrencyCache{})
 	concurrencyHelper := NewConcurrencyHelper(concurrencySvc, SSEPingFormatClaude, 0)