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

web/translation/zh-TW.json

@@ -246,7 +246,10 @@
       "importDatabaseError": "匯入資料庫時發生錯誤",
       "readDatabaseError": "讀取資料庫時發生錯誤",
       "getDatabaseError": "檢索資料庫時發生錯誤",
-      "getConfigError": "檢索設定檔時發生錯誤"
+      "getConfigError": "檢索設定檔時發生錯誤",
+      "backupPostgresNote": "此面板執行於 PostgreSQL 上。「備份」會下載一個 pg_dump 封存檔（.dump），「還原」會透過 pg_restore 重新載入。伺服器需要安裝 PostgreSQL 用戶端工具（pg_dump 與 pg_restore）。",
+      "exportDatabasePgDesc": "點擊將目前資料庫的 PostgreSQL 傾印（.dump）下載到您的裝置。",
+      "importDatabasePgDesc": "點擊選擇並上傳 .dump 檔案以還原您的 PostgreSQL 資料庫。此操作將取代所有目前的資料。"
     },
     "inbounds": {
       "title": "入站",