Add filemtime-based cache busting for styles.css and main.js

Safari caches static assets far more aggressively than Chrome/Firefox
and largely ignores heuristic revalidation, so a new deploy could keep
showing the old CSS/JS indefinitely even on a hard refresh — no
Cache-Control headers were set anywhere to guide it otherwise.

inc/assets.php adds asset_version($path), which appends a ?v=<mtime>
query string derived from the file's last-modified time. Every page
that links styles.css or main.js (index.php, menu.php, and all five
admin pages) now uses it, so the URL changes whenever the file's
contents change and browsers always fetch the new version.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

index.php

@@ -1,5 +1,6 @@
 <?php
 require __DIR__ . '/config.php';
+require __DIR__ . '/inc/assets.php';
 require __DIR__ . '/inc/google-reviews.php';
 require __DIR__ . '/inc/announcements.php';
 require __DIR__ . '/inc/hours.php';
@@ -17,7 +18,7 @@ $siteUrl = rtrim(SITE_URL, '/');
   <title>The Old Fitz Public House</title>
   <link rel="preconnect" href="https://fonts.googleapis.com">
   <link rel="preconnect" href="https://fonts.gstatic.com" crossorigin>
-  <link rel="stylesheet" href="styles.css">
+  <link rel="stylesheet" href="styles.css<?= asset_version('styles.css') ?>">
   <link href="https://fonts.googleapis.com/css2?family=Fraunces:ital,opsz,wght@0,9..144,300;0,9..144,400;0,9..144,500;0,9..144,600;0,9..144,700;1,9..144,400;1,9..144,500&family=Karla:wght@400;500;600;700&family=IBM+Plex+Mono:wght@400;500&display=swap" rel="stylesheet">
 
   <meta name="description" content="The Old Fitz Public House — a traditional Irish pub in Marion, Indiana. Open Thursday through Saturday, 4–10pm. Family friendly, no reservations.">
@@ -284,7 +285,7 @@ $siteUrl = rtrim(SITE_URL, '/');
     </div>
   </footer>
 
-  <script src="main.js" defer></script>
+  <script src="main.js<?= asset_version('main.js') ?>" defer></script>
 </body>
 
 </html>