feat(ops): 添加运维监控配置开关

- 在 .env.example 和 config.example.yaml 中添加 ops.enabled 配置项
- 默认值为 true，保持现有行为
- 当设置为 false 时，左侧栏隐藏运维监控菜单并禁用所有运维监控功能
- 修改后端 GetSettings API，让 ops_monitoring_enabled 受 config.ops.enabled 控制
- 数据清理和预聚合任务默认保持开启状态（通过运维监控设置对话框配置）

backend/internal/handler/admin/setting_handler.go

@@ -19,14 +19,16 @@ type SettingHandler struct {
 	settingService   *service.SettingService
 	emailService     *service.EmailService
 	turnstileService *service.TurnstileService
+	opsService       *service.OpsService
 }
 
 // NewSettingHandler 创建系统设置处理器
-func NewSettingHandler(settingService *service.SettingService, emailService *service.EmailService, turnstileService *service.TurnstileService) *SettingHandler {
+func NewSettingHandler(settingService *service.SettingService, emailService *service.EmailService, turnstileService *service.TurnstileService, opsService *service.OpsService) *SettingHandler {
 	return &SettingHandler{
 		settingService:   settingService,
 		emailService:     emailService,
 		turnstileService: turnstileService,
+		opsService:       opsService,
 	}
 }
 
@@ -39,6 +41,9 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		return
 	}
 
+	// Check if ops monitoring is enabled (respects config.ops.enabled)
+	opsEnabled := h.opsService != nil && h.opsService.IsMonitoringEnabled(c.Request.Context())
+
 	response.Success(c, dto.SystemSettings{
 		RegistrationEnabled:                  settings.RegistrationEnabled,
 		EmailVerifyEnabled:                   settings.EmailVerifyEnabled,
@@ -72,7 +77,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		FallbackModelAntigravity:             settings.FallbackModelAntigravity,
 		EnableIdentityPatch:                  settings.EnableIdentityPatch,
 		IdentityPatchPrompt:                  settings.IdentityPatchPrompt,
-		OpsMonitoringEnabled:                 settings.OpsMonitoringEnabled,
+		OpsMonitoringEnabled:                 opsEnabled && settings.OpsMonitoringEnabled,
 		OpsRealtimeMonitoringEnabled:         settings.OpsRealtimeMonitoringEnabled,
 		OpsQueryModeDefault:                  settings.OpsQueryModeDefault,
 		OpsMetricsIntervalSeconds:            settings.OpsMetricsIntervalSeconds,