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/controller/server.go

@@ -8,6 +8,7 @@ import (
 	"strconv"
 	"time"
 
+	"github.com/mhsanaei/3x-ui/v3/database"
 	"github.com/mhsanaei/3x-ui/v3/logger"
 	"github.com/mhsanaei/3x-ui/v3/web/entity"
 	"github.com/mhsanaei/3x-ui/v3/web/global"
@@ -279,6 +280,9 @@ func (a *ServerController) getDb(c *gin.Context) {
 	}
 
 	filename := "x-ui.db"
+	if database.IsPostgres() {
+		filename = "x-ui.dump"
+	}
 	if !filenameRegex.MatchString(filename) {
 		c.AbortWithError(http.StatusBadRequest, fmt.Errorf("invalid filename"))
 		return