fix(metrics): 在主路由中注册 /metrics 端点

- 添加 metrics 包导入
- 在主 handler 中注册 GET /metrics 路由

internal/app/http_api.go

@@ -24,6 +24,7 @@ import (
 	"sub2api-cn-relay-manager/internal/store/sqlite"
 
 	"sub2api-cn-relay-manager/internal/access"
+	"sub2api-cn-relay-manager/internal/metrics"
 )
 
 // 版本信息变量，在构建时通过 -ldflags 注入
@@ -336,6 +337,7 @@ func NewAPIHandlerWithAuth(adminAuth AdminAuthConfig, actions ActionSet) http.Ha
 	mux := http.NewServeMux()
 	mux.HandleFunc("GET /healthz", healthz)
 	mux.HandleFunc("GET /version", handleVersion)
+	mux.Handle("GET /metrics", metrics.Handler())
 	mux.HandleFunc("GET /api/admin/session", func(w http.ResponseWriter, r *http.Request) {
 		handleAdminSessionState(w, r, adminAuth)
 	})