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>

includes/admin_layout.php

@@ -0,0 +1,84 @@
+<?php
+// Shared page chrome for the admin pages: the site's Strata layout with an admin menu in the sidebar.
+
+if (basename($_SERVER['SCRIPT_FILENAME'] ?? '') === basename(__FILE__)) {
+    http_response_code(403);
+    exit;
+}
+
+function e(string $s): string {
+    return htmlspecialchars($s, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
+}
+
+function adminHeader(string $title, ?array $admin = null): void {
+?>
+<!DOCTYPE HTML>
+<html>
+	<head>
+		<title><?= e($title) ?> | RFE Admin</title>
+		<meta charset="utf-8" />
+		<meta name="viewport" content="width=device-width, initial-scale=1" />
+		<meta name="robots" content="noindex, nofollow" />
+		<link rel="stylesheet" href="../assets/css/main.css" />
+		<style>
+		        #header ul.admin-nav {
+		            list-style: none;
+		            padding: 0;
+		            margin-top: 2em;
+		        }
+		        #header ul.admin-nav li {
+		            padding: 0.25em 0;
+		        }
+		        #header ul.admin-nav a {
+		            color: #fff;
+		        }
+		        .admin-message {
+		            padding: 0.75em 1em;
+		            border-radius: 4px;
+		            margin-bottom: 2em;
+		        }
+		        .admin-message.error {
+		            background-color: rgba(200, 60, 60, 0.15);
+		            color: #a33;
+		        }
+		        .admin-message.success {
+		            background-color: rgba(60, 160, 90, 0.15);
+		            color: #2a7a45;
+		        }
+		</style>
+	</head>
+	<body>
+		<header id="header">
+			<div class="inner">
+				<a href="../index.php" class="image avatar"><img src="../images/avatar-rfe.jpg" alt="Radio Free Elsewhere logo" /></a>
+				<h1><strong>Radio Free Elsewhere</strong><br>
+				Admin</h1>
+<?php if ($admin): ?>
+				<ul class="admin-nav">
+					<li><a href="index.php">Dashboard</a></li>
+					<li><a href="../live.php">View live page</a></li>
+				</ul>
+				<form method="post" action="logout.php">
+					<?= csrfField() ?>
+					<input type="submit" class="small" value="Log out <?= e($admin['username']) ?>" />
+				</form>
+<?php endif; ?>
+			</div>
+		</header>
+
+		<div id="main">
+			<section>
+				<header class="major">
+					<h2><?= e($title) ?></h2>
+				</header>
+<?php
+}
+
+function adminFooter(): void {
+?>
+			</section>
+		</div>
+	</body>
+</html>
+<?php
+}