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/index.php

@@ -0,0 +1,27 @@
+<?php
+require __DIR__ . '/../includes/auth.php';
+require __DIR__ . '/../includes/admin_layout.php';
+require __DIR__ . '/../includes/broadcasts.php';
+
+$admin = requireAdmin();
+$db = getDB();
+
+$live = liveBroadcast($db);
+$show = currentShow($db);
+
+adminHeader('Dashboard', $admin);
+?>
+				<p>Logged in as <strong><?= e($admin['username']) ?></strong>.</p>
+
+				<h3>Broadcast status</h3>
+				<p>
+<?php if ($live): ?>
+					Live now<?= $show ? ': <strong>' . e($show['name']) . '</strong>' : ' (no show set)' ?>, since <?= e($live['started_at']) ?> UTC.
+<?php elseif ($show): ?>
+					Offline. <strong><?= e($show['name']) ?></strong> is queued for the next broadcast.
+<?php else: ?>
+					Offline. No show is queued.
+<?php endif; ?>
+				</p>
+<?php
+adminFooter();