feat(monitoring): add node/tunnel metrics, service monitors, and health checks

- Add NodeMetric/TunnelMetric/ServiceMonitor models and repository methods
- Implement metrics ingestion service with per-minute bucket aggregation
- Add health checker for node connectivity monitoring
- Wire node metrics from WebSocket SystemInfo messages
- Add tunnel metrics ingestion from flow upload endpoint
- Create monitoring REST API endpoints for nodes, tunnels, services
- Implement service monitor CRUD and execution (TCP/ICMP checks)
- Add MonitorPermission for non-admin access control
- Create frontend monitor page with node/tunnel/service views
- Add tunnel metrics ingestion from agent flow reports
- Include schema migration for tunnel_metric unique index
- Fix tunnel entry port conflict validation to use transaction

Entire-Checkpoint: 030821a7c8e3

go-backend/internal/http/handler/jobs.go

@@ -18,12 +18,14 @@ func (h *Handler) StartBackgroundJobs() {
 	ctx, cancel := context.WithCancel(context.Background())
 	h.jobsCancel = cancel
 	h.jobsStarted = true
-	h.jobsWG.Add(3)
+	h.jobsWG.Add(5)
 	h.jobsMu.Unlock()
 
 	go h.runHourlyStatsLoop(ctx)
 	go h.runDailyMaintenanceLoop(ctx)
 	go h.runNodeRenewalCycleLoop(ctx)
+	go h.runMetricsIngestion(ctx)
+	go h.runHealthChecks(ctx)
 }
 
 func (h *Handler) StopBackgroundJobs() {
@@ -47,6 +49,20 @@ func (h *Handler) StopBackgroundJobs() {
 	h.jobsWG.Wait()
 }
 
+func (h *Handler) runMetricsIngestion(ctx context.Context) {
+	defer h.jobsWG.Done()
+	if h.metrics != nil {
+		h.metrics.Start(ctx)
+	}
+}
+
+func (h *Handler) runHealthChecks(ctx context.Context) {
+	defer h.jobsWG.Done()
+	if h.healthCheck != nil {
+		h.healthCheck.Start(ctx)
+	}
+}
+
 func (h *Handler) runHourlyStatsLoop(ctx context.Context) {
 	defer h.jobsWG.Done()