fix(backend): secure production startup and migrations

backend/internal/models/postgres_integration_test.go

@@ -3,20 +3,15 @@
 package models
 
 import (
-	"os"
 	"testing"
 
 	"github.com/stretchr/testify/require"
-	"gorm.io/driver/postgres"
 	"gorm.io/gorm"
+	"senlinai-agent/backend/internal/testutil"
 )
 
 func TestPostgresPing(t *testing.T) {
-	dsn := os.Getenv("TEST_DATABASE_URL")
-	require.NotEmpty(t, dsn, "TEST_DATABASE_URL is required for integration tests and must point to an isolated database")
-
-	database, err := gorm.Open(postgres.Open(dsn), &gorm.Config{TranslateError: true})
-	require.NoError(t, err)
+	database := testutil.OpenIsolatedPostgres(t, &gorm.Config{TranslateError: true})
 	sqlDB, err := database.DB()
 	require.NoError(t, err)
 	require.NoError(t, sqlDB.Ping())