feat(notify): 告警补服务端掉线/Agent 不可达/配置漂移，巡检改 Source 接口（v1.2.10）

- 三个新事件（默认关闭）：公网服务端掉线、公网 agent 不可达、配置漂移；连续失败阈值与漂移提醒间隔均可配，达标与恢复各报一次

- 避免误报：只在「用户要服务端运行」时判定掉线；ConnectionState=unknown 按健康处理；没配 agent 的服务端不参与可达性判定

- internal/monitor 改 Source 接口（Routes/Servers/Agents/Drift），frp 适配在 internal/app/monitor_source.go；「连续 N 次」抽成共用 failureTransition，通知一律在锁外调用

- 通知图标分类补齐；通知方式标签改「Webhook / IM 机器人」；新增 6 个设置项；补 serverDown/serverLabel/stateOr 测试

- agent 无改动，AgentVersion 保持 0.12.2

internal/settings/settings.go

@@ -46,6 +46,12 @@ const (
 	KeyNotifyMemoryThreshold       = "notify_memory_threshold"
 	KeyNotifyDiskThreshold         = "notify_disk_threshold"
 	KeyNotifyTunnelFailThreshold   = "notify_tunnel_fail_threshold"
+	KeyNotifyOnFrpsDown            = "notify_on_frps_down"
+	KeyNotifyOnAgentUnreachable    = "notify_on_agent_unreachable"
+	KeyNotifyOnRouteDrift          = "notify_on_route_drift"
+	KeyNotifyFrpsFailThreshold     = "notify_frps_fail_threshold"
+	KeyNotifyAgentFailThreshold    = "notify_agent_fail_threshold"
+	KeyNotifyDriftCooldownHours    = "notify_drift_cooldown_hours"
 	KeyTrustedProxy                = "trusted_proxy_json"
 	KeyGlobalIPBlacklist           = "global_ip_blacklist"
 	KeyGlobalIPWhitelist           = "global_ip_whitelist"