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

.vscode/launch.json

@@ -17,5 +17,22 @@
       },
       "console": "integratedTerminal"
     },
+    {
+      "name": "Run 3x-ui (Postgres)",
+      "type": "go",
+      "request": "launch",
+      "mode": "auto",
+      "program": "${workspaceFolder}",
+      "cwd": "${workspaceFolder}",
+      "env": {
+        "XUI_DEBUG": "true",
+        "XUI_LOG_FOLDER": "x-ui",
+        "XUI_BIN_FOLDER": "x-ui",
+        "XUI_DB_TYPE": "postgres",
+        "XUI_DB_DSN": "postgres://xui:xuipass@127.0.0.1:5432/xui?sslmode=disable",
+        "PATH": "C:\\Program Files\\PostgreSQL\\18\\bin;${env:PATH}"
+      },
+      "console": "integratedTerminal"
+    },
   ]
 }