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

install.sh

@@ -218,6 +218,41 @@ EOF
     return 0
 }
 
+ensure_pg_client() {
+    if command -v pg_dump > /dev/null 2>&1 && command -v pg_restore > /dev/null 2>&1; then
+        return 0
+    fi
+    echo -e "${yellow}Installing PostgreSQL client tools (pg_dump/pg_restore) for in-panel backup...${plain}" >&2
+    case "${release}" in
+        ubuntu | debian | armbian)
+            apt-get update >&2 && apt-get install -y -q postgresql-client >&2 || return 1
+            ;;
+        fedora | amzn | virtuozzo | rhel | almalinux | rocky | ol)
+            dnf install -y -q postgresql >&2 || return 1
+            ;;
+        centos)
+            if [[ "${VERSION_ID}" =~ ^7 ]]; then
+                yum install -y postgresql >&2 || return 1
+            else
+                dnf install -y -q postgresql >&2 || return 1
+            fi
+            ;;
+        arch | manjaro | parch)
+            pacman -Sy --noconfirm postgresql >&2 || return 1
+            ;;
+        opensuse-tumbleweed | opensuse-leap)
+            zypper -q install -y postgresql >&2 || return 1
+            ;;
+        alpine)
+            apk add --no-cache postgresql-client >&2 || return 1
+            ;;
+        *)
+            return 1
+            ;;
+    esac
+    command -v pg_dump > /dev/null 2>&1 && command -v pg_restore > /dev/null 2>&1
+}
+
 install_acme() {
     echo -e "${green}Installing acme.sh for SSL certificate management...${plain}"
     cd ~ || return 1
@@ -941,6 +976,7 @@ EOF
                     umask 022
                     export XUI_DB_TYPE=postgres
                     export XUI_DB_DSN="${xui_dsn}"
+                    ensure_pg_client || echo -e "${yellow}⚠ Could not install pg_dump/pg_restore. In-panel database backup/restore will be unavailable until you install the postgresql-client package.${plain}"
                 fi
             fi
 
@@ -989,6 +1025,13 @@ EOF
                 echo -e "${yellow}⚠ SSL Certificate: Skipped — panel is HTTP-only. Use a reverse proxy or SSH tunnel.${plain}"
             fi
 
+            if [[ "$db_choice" == "2" ]]; then
+                echo ""
+                echo -e "${green}PostgreSQL backup & restore is built into the panel:${plain}"
+                echo -e "  ${blue}${SSL_SCHEME}://${SSL_HOST}:${config_port}/${config_webBasePath}${plain} → Backup & Restore"
+                echo -e "${yellow}  Back Up downloads a pg_dump .dump file; Restore reloads it via pg_restore.${plain}"
+            fi
+
             if [[ "$db_choice" == "2" && "$pg_local_installed" == "1" ]]; then
                 echo ""
                 echo -e "${green}═══════════════════════════════════════════${plain}"