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/external_hwid_test.go

@@ -6,17 +6,14 @@ import (
 	"path/filepath"
 	"testing"
 
-	"github.com/mhsanaei/3x-ui/v3/internal/database"
+	"github.com/mhsanaei/3x-ui/v3/internal/database/dbtest"
 	"github.com/mhsanaei/3x-ui/v3/internal/web/service"
 )
 
 // #6559: the Master panel must send a stable X-HWID when fetching external
 // subscriptions, otherwise an HWID-limited donor answers 404.
 func TestFetchSendsStableHwid(t *testing.T) {
-	if err := database.InitDB(filepath.Join(t.TempDir(), "x-ui.db")); err != nil {
-		t.Fatalf("InitDB: %v", err)
-	}
-	t.Cleanup(func() { _ = database.CloseDB() })
+	dbtest.InitDB(t, filepath.Join(t.TempDir(), "x-ui.db"))
 
 	var gotHwid string
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {