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/setting_service.go

@@ -1060,6 +1060,7 @@ func (s *SettingService) buildSystemSettingsUpdates(ctx context.Context, setting
 	// 默认配置
 	updates[SettingKeyDefaultConcurrency] = strconv.Itoa(settings.DefaultConcurrency)
 	updates[SettingKeyDefaultBalance] = strconv.FormatFloat(settings.DefaultBalance, 'f', 8, 64)
+	updates[SettingKeyDefaultUserRPMLimit] = strconv.Itoa(settings.DefaultUserRPMLimit)
 	defaultSubsJSON, err := json.Marshal(settings.DefaultSubscriptions)
 	if err != nil {
 		return nil, fmt.Errorf("marshal default subscriptions: %w", err)
@@ -1422,6 +1423,18 @@ func (s *SettingService) GetDefaultBalance(ctx context.Context) float64 {
 	return s.cfg.Default.UserBalance
 }
 
+// GetDefaultUserRPMLimit 获取新用户默认 RPM 限制（0 = 不限制）。未配置则返回 0。
+func (s *SettingService) GetDefaultUserRPMLimit(ctx context.Context) int {
+	value, err := s.settingRepo.GetValue(ctx, SettingKeyDefaultUserRPMLimit)
+	if err != nil || value == "" {
+		return 0
+	}
+	if v, err := strconv.Atoi(value); err == nil && v >= 0 {
+		return v
+	}
+	return 0
+}
+
 // GetDefaultSubscriptions 获取新用户默认订阅配置列表。
 func (s *SettingService) GetDefaultSubscriptions(ctx context.Context) []DefaultSubscriptionSetting {
 	value, err := s.settingRepo.GetValue(ctx, SettingKeyDefaultSubscriptions)
@@ -1590,6 +1603,7 @@ func (s *SettingService) InitializeDefaultSettings(ctx context.Context) error {
 		SettingKeyOIDCConnectUserInfoUsernamePath:          "",
 		SettingKeyDefaultConcurrency:                       strconv.Itoa(s.cfg.Default.UserConcurrency),
 		SettingKeyDefaultBalance:                           strconv.FormatFloat(s.cfg.Default.UserBalance, 'f', 8, 64),
+		SettingKeyDefaultUserRPMLimit:                      "0",
 		SettingKeyDefaultSubscriptions:                     "[]",
 		SettingKeyAuthSourceDefaultEmailBalance:            "0",
 		SettingKeyAuthSourceDefaultEmailConcurrency:        "5",
@@ -1699,6 +1713,10 @@ func (s *SettingService) parseSettings(settings map[string]string) *SystemSettin
 		result.DefaultConcurrency = s.cfg.Default.UserConcurrency
 	}
 
+	if rpm, err := strconv.Atoi(settings[SettingKeyDefaultUserRPMLimit]); err == nil && rpm >= 0 {
+		result.DefaultUserRPMLimit = rpm
+	}
+
 	// 解析浮点数类型
 	if balance, err := strconv.ParseFloat(settings[SettingKeyDefaultBalance], 64); err == nil {
 		result.DefaultBalance = balance