feat(postgres): in-panel backup/restore and consistent CLI backend

Two PostgreSQL gaps on the panel:

1. x-ui setting and other CLI subcommands read XUI_DB_TYPE/XUI_DB_DSN from
   the process environment, which systemd injects via EnvironmentFile but a
   plain shell invocation does not. On a PostgreSQL install the CLI silently
   fell back to SQLite, so changes made from the management menu never
   reached the panel's database. Load the systemd EnvironmentFile
   (/etc/default/x-ui and distro equivalents) at startup; godotenv.Load does
   not override existing vars, so it stays a no-op for the managed service.

2. DB backup/restore (panel endpoints and the Telegram bot) only handled the
   SQLite file, so on PostgreSQL Back Up returned a stale/absent x-ui.db and
   Restore silently did nothing. Add pg_dump/pg_restore based backup/restore:
   - GetDb/ImportDB run pg_dump (custom format) / pg_restore, passing
     credentials via the PG* environment instead of argv.
   - getDb downloads x-ui.dump on Postgres, x-ui.db on SQLite.
   - Telegram backup sends the matching file via GetDb.
   - BackupModal shows a Postgres note and accepts .dump; the dist page
     injects window.X_UI_DB_TYPE; new strings translated for all locales.
   - install.sh installs postgresql-client for the external-DSN path and
     points the user to in-panel Backup & Restore.

Closes #4658

config/config.go

@@ -121,6 +121,19 @@ func GetDBDSN() string {
 	return strings.TrimSpace(os.Getenv("XUI_DB_DSN"))
 }
 
+// GetEnvFilePaths returns the candidate service environment file paths (the file
+// systemd loads via EnvironmentFile) across the supported distro families.
+func GetEnvFilePaths() []string {
+	if runtime.GOOS == "windows" {
+		return nil
+	}
+	return []string{
+		"/etc/default/x-ui",
+		"/etc/conf.d/x-ui",
+		"/etc/sysconfig/x-ui",
+	}
+}
+
 // GetLogFolder returns the path to the log folder based on environment variables or platform defaults.
 func GetLogFolder() string {
 	logFolderPath := os.Getenv("XUI_LOG_FOLDER")