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

main.go

@@ -432,9 +432,27 @@ func migrateDb() {
 	fmt.Println("Migration done!")
 }
 
+// loadServiceEnvFile loads the systemd EnvironmentFile so CLI subcommands like
+// "x-ui setting" hit the same database backend as the panel. godotenv.Load does
+// not override variables already in the environment, so it is a no-op for the
+// systemd-managed service.
+func loadServiceEnvFile() {
+	for _, path := range config.GetEnvFilePaths() {
+		if _, err := os.Stat(path); err != nil {
+			continue
+		}
+		if err := godotenv.Load(path); err != nil {
+			log.Printf("warning: failed to load env file %s: %v", path, err)
+		}
+		return
+	}
+}
+
 // main is the entry point of the 3x-ui application.
 // It parses command-line arguments to run the web server, migrate database, or update settings.
 func main() {
+	loadServiceEnvFile()
+
 	if len(os.Args) < 2 {
 		runWebServer()
 		return