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/admin/setting_handler.go

@@ -185,6 +185,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		CustomEndpoints:                        dto.ParseCustomEndpoints(settings.CustomEndpoints),
 		DefaultConcurrency:                     settings.DefaultConcurrency,
 		DefaultBalance:                         settings.DefaultBalance,
+		DefaultUserRPMLimit:                    settings.DefaultUserRPMLimit,
 		DefaultSubscriptions:                   defaultSubscriptions,
 		EnableModelFallback:                    settings.EnableModelFallback,
 		FallbackModelAnthropic:                 settings.FallbackModelAnthropic,
@@ -332,6 +333,7 @@ type UpdateSettingsRequest struct {
 	// 默认配置
 	DefaultConcurrency                       int                               `json:"default_concurrency"`
 	DefaultBalance                           float64                           `json:"default_balance"`
+	DefaultUserRPMLimit                      int                               `json:"default_user_rpm_limit"`
 	DefaultSubscriptions                     []dto.DefaultSubscriptionSetting  `json:"default_subscriptions"`
 	AuthSourceDefaultEmailBalance            *float64                          `json:"auth_source_default_email_balance"`
 	AuthSourceDefaultEmailConcurrency        *int                              `json:"auth_source_default_email_concurrency"`
@@ -1105,6 +1107,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		CustomEndpoints:                  customEndpointsJSON,
 		DefaultConcurrency:               req.DefaultConcurrency,
 		DefaultBalance:                   req.DefaultBalance,
+		DefaultUserRPMLimit:              req.DefaultUserRPMLimit,
 		DefaultSubscriptions:             defaultSubscriptions,
 		EnableModelFallback:              req.EnableModelFallback,
 		FallbackModelAnthropic:           req.FallbackModelAnthropic,
@@ -1400,6 +1403,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		CustomEndpoints:                        dto.ParseCustomEndpoints(updatedSettings.CustomEndpoints),
 		DefaultConcurrency:                     updatedSettings.DefaultConcurrency,
 		DefaultBalance:                         updatedSettings.DefaultBalance,
+		DefaultUserRPMLimit:                    updatedSettings.DefaultUserRPMLimit,
 		DefaultSubscriptions:                   updatedDefaultSubscriptions,
 		EnableModelFallback:                    updatedSettings.EnableModelFallback,
 		FallbackModelAnthropic:                 updatedSettings.FallbackModelAnthropic,