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/database/dbtest/dbtest_test.go

@@ -0,0 +1,36 @@
+package dbtest
+
+import (
+	"path/filepath"
+	"testing"
+
+	"github.com/mhsanaei/3x-ui/v3/internal/database"
+	"github.com/mhsanaei/3x-ui/v3/internal/database/model"
+)
+
+func TestInitDBGivesEachTestItsOwnDatabase(t *testing.T) {
+	t.Run("first test writes", func(t *testing.T) {
+		InitDB(t, filepath.Join(t.TempDir(), "x-ui.db"))
+		if err := database.GetDB().Create(&model.Setting{Key: "dbtestProbe", Value: "first"}).Error; err != nil {
+			t.Fatalf("write probe: %v", err)
+		}
+	})
+
+	t.Run("next test starts clean", func(t *testing.T) {
+		InitDB(t, filepath.Join(t.TempDir(), "x-ui.db"))
+		var leaked int64
+		if err := database.GetDB().Model(&model.Setting{}).Where("key = ?", "dbtestProbe").Count(&leaked).Error; err != nil {
+			t.Fatalf("count probe: %v", err)
+		}
+		if leaked != 0 {
+			t.Fatalf("database holds %d probe rows written by the previous test; copies must not share state", leaked)
+		}
+		var admins int64
+		if err := database.GetDB().Model(&model.User{}).Count(&admins).Error; err != nil {
+			t.Fatalf("count users: %v", err)
+		}
+		if admins != 1 {
+			t.Fatalf("users = %d, want the 1 seeded admin a fresh install has", admins)
+		}
+	})
+}