fix: resolve 3 billing admin errors and normalize remote_servers query

- admin_config.php: guard session_start() — was firing Notice because
  admin_auth.php → session_bridge.php already started the session
- includes/menu.php: check mysqli_thread_id() before reusing $db so a
  closed handle does not cause 'mysqli object is already closed' fatal
- admin_invoices.php / admin_payments.php: set $db = null after
  mysqli_close() so menu.php's reuse-check correctly falls through to
  opening a fresh connection
- adminserverlist.php: use col_exists() to detect missing 'enabled'
  column in gsp_remote_servers; fall back to constant 1 and display a
  schema-notice banner; guard UPDATE accordingly; also add missing
  price_daily / price_year columns to the services SELECT; remove
  duplicate 'Update Enabled Servers' button
- add_remote_server_enabled_column.sql: idempotent migration to add the
  'enabled' INT column to gsp_remote_servers on older installs

Agent-Logs-Url: https://github.com/GameServerPanel/GSP/sessions/988997ed-7568-48bf-96ef-889fb5d91fec

Co-authored-by: iaretechnician <2749183+iaretechnician@users.noreply.github.com>

modules/billing/admin_invoices.php

@@ -66,6 +66,7 @@ if ($_SERVER['REQUEST_METHOD'] === 'POST' && isset($_POST['action'], $_POST['inv
     if (!headers_sent()) {
         header('Location: admin_invoices.php?msg=' . urlencode($message) . '&type=' . $msgType);
         mysqli_close($db);
+        $db = null;
         exit;
     }
 }
@@ -81,6 +82,7 @@ $res = $db->query(
 );
 if ($res) $invoices = $res->fetch_all(MYSQLI_ASSOC);
 mysqli_close($db);
+$db = null;
 
 if (isset($_GET['msg']))  $message = $_GET['msg'];
 if (isset($_GET['type'])) $msgType = $_GET['type'];