feat: add background music support and static files mount

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/player.html

@@ -345,6 +345,7 @@ function handleMessage(msg) {
     show('results-screen');
   }
   else if (msg.type === 'podium') {
+    stopBGM();
     stopGame();
     showPodium(msg.leaderboard);
   }
@@ -367,6 +368,7 @@ document.getElementById('join-btn').addEventListener('click', () => {
   }
   myDept = dept;
   const displayName = `${fn} ${ln}`;
+  startBGM();
   ws.send(JSON.stringify({type: 'join', name: `${fn} ${ln}`, dept, display: displayName}));
 });
 
@@ -749,6 +751,21 @@ function hexToRgb(hex) {
   return `${r},${g},${b}`;
 }
 
+// ── BACKGROUND MUSIC ─────────────────────────────────────────────────────────
+const bgm = document.getElementById('bgm');
+let bgmStarted = false;
+function startBGM() {
+  if (bgmStarted) return;
+  bgmStarted = true;
+  bgm.volume = 0.35;
+  bgm.play().catch(() => {});
+}
+function stopBGM() {
+  bgm.pause();
+  bgm.currentTime = 0;
+  bgmStarted = false;
+}
+
 connectWS();
 
 function exitGame() {
@@ -758,5 +775,6 @@ function exitGame() {
   connectWS();
 }
 </script>
+<audio id="bgm" src="/static/music.mp3" loop preload="auto"></audio>
 </body>
 </html>