feat: make scoreboard playoff-aware with banner, bracket, and series drill-down

Turn a regular-season-looking Tuesday into a full playoff experience:

- Playoff banner with round + day + series + elimination counts, gold/silver
  Cup theme toggled by body.playoff-mode
- Series context on each playoff card: round chip, series score, stake badges
  (GAME 7, CLINCHER, PIVOTAL), and one-line blurb
- Game 7s pin to a new Spotlight section above Live
- Playoff OT renders with SUDDEN DEATH badge and pulsing gold border
- Client-side OT notifications via bell button in the banner
- New /series/<id> drill-down with headline, next-game, and game-by-game history
- New /bracket page: 7-column desktop grid, accordion on mobile
- Day N banner count auto-anchors on first playoff scoreboard hit
- SQLite cache for bracket + per-series schedules, stale-on-failure up to 24h

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/static/sw.js

@@ -1,4 +1,4 @@
-const CACHE = 'nhl-scoreboard-v1';
+const CACHE = 'nhl-scoreboard-v3';
 const PRECACHE = [
     '/',
     '/static/styles.css',
@@ -33,6 +33,20 @@ self.addEventListener('fetch', event => {
         return;
     }
 
+    // Network-first for bracket + series detail pages; fall back to cache offline
+    if (pathname === '/bracket' || pathname.startsWith('/series/')) {
+        event.respondWith(
+            fetch(event.request).then(response => {
+                if (response.ok) {
+                    const clone = response.clone();
+                    caches.open(CACHE).then(c => c.put(event.request, clone));
+                }
+                return response;
+            }).catch(() => caches.match(event.request))
+        );
+        return;
+    }
+
     // Cache-first for everything else (static assets, shell)
     event.respondWith(
         caches.match(event.request).then(cached => {