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/web/service/client_hwid_tx_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"
 
 	"gorm.io/gorm"
@@ -188,10 +189,7 @@ func TestSetClientLimitHwidIsSerializedWithSyncInbound(t *testing.T) {
 func BenchmarkSetClientLimitHwidSerialized(b *testing.B) {
 	dbDir := b.TempDir()
 	b.Setenv("XUI_DB_FOLDER", dbDir)
-	if err := database.InitDB(filepath.Join(dbDir, "x-ui.db")); err != nil {
-		b.Fatalf("InitDB: %v", err)
-	}
-	b.Cleanup(func() { _ = database.CloseDB() })
+	dbtest.InitDB(b, filepath.Join(dbDir, "x-ui.db"))
 	StartTrafficWriter()
 	b.Cleanup(StopTrafficWriter)
 	db := database.GetDB()