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/sub_scale_test.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/mhsanaei/3x-ui/v3/internal/config"
 	"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"
 	xuilogger "github.com/mhsanaei/3x-ui/v3/internal/logger"
 	"github.com/mhsanaei/3x-ui/v3/internal/xray"
@@ -38,10 +39,7 @@ func setupScaleSubDB(t *testing.T) {
 	}
 	switch strings.ToLower(strings.TrimSpace(os.Getenv("XUI_SCALE_TEST"))) {
 	case "1", "true", "yes":
-		if err := database.InitDB(filepath.Join(t.TempDir(), "scale.db")); err != nil {
-			t.Fatalf("InitDB(sqlite): %v", err)
-		}
-		t.Cleanup(func() { _ = database.CloseDB() })
+		dbtest.InitDB(t, filepath.Join(t.TempDir(), "scale.db"))
 		return
 	}
 	t.Skip("set XUI_SCALE_TEST=1 (sqlite) or XUI_DB_TYPE=postgres + XUI_DB_DSN (postgres) to run the scale benchmark")