perf(tests): copy a migrated template DB instead of migrating per test

Most tests opened a throwaway panel DB with database.InitDB, which runs the
full AutoMigrate + seed on an empty file every time: ~230ms, and ~850ms under
-race because GORM's reflection-heavy migration is what the detector slows
most. internal/web/service does this in ~550 of its 830 tests, so the CI race
job spent ~10 of its ~14.6 minutes re-migrating empty databases.

internal/database/dbtest.InitDB migrates once per test process, then hands
each test its own copy of that file (~130ms under -race) and registers the
CloseDB cleanup. The copy then goes through InitDB like a panel restart, so
every test still starts from the state a fresh install has. Tests that reopen
an existing file, migrate a hand-built legacy DB or target Postgres keep
calling database.InitDB.

Locally under -race: internal/web/service 626s (last CI run) -> 114s,
internal/sub 246s -> 35s.

internal/sub/service_info_node_test.go

@@ -8,6 +8,7 @@ import (
 	"time"
 
 	"github.com/mhsanaei/3x-ui/v3/internal/database"
+	"github.com/mhsanaei/3x-ui/v3/internal/database/dbtest"
 	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
 	"github.com/mhsanaei/3x-ui/v3/internal/xray"
@@ -15,12 +16,7 @@ import (
 
 func setupInfoNodeTestDB(t *testing.T) {
 	t.Helper()
-	if err := database.InitDB(t.TempDir() + "/test_infonode.db"); err != nil {
-		t.Fatalf("InitDB: %v", err)
-	}
-	t.Cleanup(func() {
-		_ = database.CloseDB()
-	})
+	dbtest.InitDB(t, t.TempDir()+"/test_infonode.db")
 	db := database.GetDB()
 	if err := db.AutoMigrate(
 		&model.Inbound{},