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/middleware/auth.go

@@ -101,6 +101,10 @@ func shouldSkip(path string) bool {
 }
 
 func requiresAdmin(path string) bool {
+	if strings.HasPrefix(path, "/api/v1/monitor/permission/") {
+		return true
+	}
+
 	if strings.HasPrefix(path, "/api/v1/group/") {
 		return true
 	}