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-gost/x/service/service.go

@@ -7,7 +7,6 @@ import (
 	"errors"
 	"fmt"
 	"io"
-	"log"
 	"net"
 	"os"
 	"os/exec"
@@ -63,11 +62,9 @@ func SetProtocolBlock(httpOn int, tlsOn int, socksOn int) {
 type Option func(opts *options)
 
 func init() {
-	_, err := LoadConfig("config.json")
-	fmt.Println("config.json loaded")
-	if err != nil {
-		log.Fatal(err)
-	}
+	// NOTE: This package can be imported by tests/tools that don't have a local
+	// config.json. Missing config should not crash the process.
+	_, _ = LoadConfig("config.json")
 	needWrap = isTls+isSocks+isHttp > 0
 }