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

frontend/src/env.d.ts

@@ -26,6 +26,7 @@ interface SubPageData {
 interface Window {
   X_UI_BASE_PATH?: string;
   X_UI_CUR_VER?: string;
+  X_UI_DB_TYPE?: string;
   __SUB_PAGE_DATA__?: SubPageData;
 }
 

frontend/src/pages/index/BackupModal.tsx

@@ -19,6 +19,7 @@ interface BackupModalProps {
 
 export default function BackupModal({ open, basePath: _basePath, onClose, onBusy }: BackupModalProps) {
   const { t } = useTranslation();
+  const isPostgres = window.X_UI_DB_TYPE === 'postgres';
 
   function exportDb() {
     window.location.href = (window.X_UI_BASE_PATH || '') + 'panel/api/server/getDb';
@@ -27,7 +28,7 @@ export default function BackupModal({ open, basePath: _basePath, onClose, onBusy
   function importDb() {
     const fileInput = document.createElement('input');
     fileInput.type = 'file';
-    fileInput.accept = '.db';
+    fileInput.accept = isPostgres ? '.dump' : '.db';
     fileInput.addEventListener('change', async (e) => {
       const dbFile = (e.target as HTMLInputElement).files?.[0];
       if (!dbFile) return;
@@ -65,11 +66,18 @@ export default function BackupModal({ open, basePath: _basePath, onClose, onBusy
       footer={null}
       onCancel={onClose}
     >
+      {isPostgres && (
+        <div className="backup-description" style={{ marginBottom: 16 }}>
+          {t('pages.index.backupPostgresNote')}
+        </div>
+      )}
       <div className="backup-list">
         <div className="backup-item">
           <div className="backup-meta">
             <div className="backup-title">{t('pages.index.exportDatabase')}</div>
-            <div className="backup-description">{t('pages.index.exportDatabaseDesc')}</div>
+            <div className="backup-description">
+              {isPostgres ? t('pages.index.exportDatabasePgDesc') : t('pages.index.exportDatabaseDesc')}
+            </div>
           </div>
           <Button type="primary" onClick={exportDb} icon={<DownloadOutlined />} />
         </div>
@@ -77,7 +85,9 @@ export default function BackupModal({ open, basePath: _basePath, onClose, onBusy
         <div className="backup-item">
           <div className="backup-meta">
             <div className="backup-title">{t('pages.index.importDatabase')}</div>
-            <div className="backup-description">{t('pages.index.importDatabaseDesc')}</div>
+            <div className="backup-description">
+              {isPostgres ? t('pages.index.importDatabasePgDesc') : t('pages.index.importDatabaseDesc')}
+            </div>
           </div>
           <Button type="primary" onClick={importDb} icon={<UploadOutlined />} />
         </div>