Add admin login

- admins and login_attempts tables
- cli/create_admin.php: create an admin or reset a password over SSH
  (no email involved)
- includes/auth.php: hardened admin sessions, login throttling (5
  failures per IP per 15 minutes) and CSRF helpers
- admin/: login, logout and a dashboard showing broadcast status
- includes/admin_layout.php: shared admin page layout

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

admin/logout.php

@@ -0,0 +1,11 @@
+<?php
+require __DIR__ . '/../includes/auth.php';
+
+// Logout is POST-only with a CSRF token, so other sites can't log you out with a link or image
+if ($_SERVER['REQUEST_METHOD'] === 'POST') {
+    verifyCsrf();
+    startAdminSession();
+    logoutAdmin();
+}
+
+header('Location: login.php');