chore(db): use DELETE journal mode so sqlite stays a single file

Switch sqlite from WAL to DELETE journal mode so the database no longer
keeps -shm/-wal sidecar files; only x-ui.db remains at rest. Pair with
synchronous=FULL for crash-safe durability in rollback-journal mode.

The startup PRAGMA journal_mode=DELETE converts existing WAL databases
and removes their leftover sidecar files on first run, so upgrades need
no manual cleanup. busy_timeout and _txlock=immediate are unchanged.

internal/database/db.go

@@ -886,7 +886,7 @@ func InitDB(dbPath string) error {
 		if err = os.MkdirAll(dir, 0755); err != nil {
 			return err
 		}
-		dsn := dbPath + "?_journal_mode=WAL&_busy_timeout=10000&_synchronous=NORMAL&_txlock=immediate"
+		dsn := dbPath + "?_journal_mode=DELETE&_busy_timeout=10000&_synchronous=FULL&_txlock=immediate"
 		db, err = gorm.Open(sqlite.Open(dsn), c)
 		if err != nil {
 			return err
@@ -895,13 +895,13 @@ func InitDB(dbPath string) error {
 		if err != nil {
 			return err
 		}
-		if _, err := sqlDB.Exec("PRAGMA journal_mode=WAL"); err != nil {
+		if _, err := sqlDB.Exec("PRAGMA journal_mode=DELETE"); err != nil {
 			return err
 		}
 		if _, err := sqlDB.Exec("PRAGMA busy_timeout=10000"); err != nil {
 			return err
 		}
-		if _, err := sqlDB.Exec("PRAGMA synchronous=NORMAL"); err != nil {
+		if _, err := sqlDB.Exec("PRAGMA synchronous=FULL"); err != nil {
 			return err
 		}
 	}