feat: auto-reload grid when background cache refresh completes

Poll /api/cache-ts every 30s; silently reload concerts if the
cache timestamp increased (background scrape finished).

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

static/app.js

@@ -449,8 +449,25 @@ document.addEventListener('keydown', e => {
   }
 });
 
+// ── Cache auto-update polling ─────────────────────────────────────────────────
+let _cacheTs = 0;
+
+async function pollCacheTs() {
+  try {
+    const { ts, count } = await fetch('/api/cache-ts').then(r => r.json());
+    if (_cacheTs && ts > _cacheTs && count > 0) {
+      _cacheTs = ts;
+      await refresh();
+    } else if (!_cacheTs) {
+      _cacheTs = ts;
+    }
+  } catch {}
+}
+
 // ── Init ─────────────────────────────────────────────────────────────────────
 (async () => {
   await Promise.all([loadCategories(), refreshDlHistory()]);
   await refresh();
+  _cacheTs = (await fetch('/api/cache-ts').then(r => r.json()).catch(() => ({ts:0}))).ts;
+  setInterval(pollCacheTs, 30_000);
 })();