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/templates/index.html

@@ -16,7 +16,42 @@
     <header>
         <span class="header-title">NHL Scoreboard</span>
     </header>
+    <section id="playoff-banner" class="playoff-banner hidden" aria-hidden="true">
+        <svg class="banner-trophy" viewBox="0 0 32 40" aria-hidden="true">
+            <defs>
+                <linearGradient id="cup-gold" x1="0" x2="1" y1="0" y2="1">
+                    <stop offset="0%" stop-color="#f5d76e"/>
+                    <stop offset="60%" stop-color="#d4af37"/>
+                    <stop offset="100%" stop-color="#8a6d1a"/>
+                </linearGradient>
+            </defs>
+            <path fill="url(#cup-gold)" d="M6 2h20v4c0 5-2 9-5 11l-1 5h-8l-1-5C8 15 6 11 6 6V2zm4 20h12v3H10v-3zm-2 4h16v3H8v-3zm1 4h14v6H9v-6z"/>
+            <rect x="11" y="9" width="10" height="2" fill="#0a1628" opacity="0.35"/>
+            <rect x="11" y="13" width="10" height="1.5" fill="#0a1628" opacity="0.35"/>
+        </svg>
+        <div class="banner-text">
+            <div class="banner-title">
+                STANLEY CUP PLAYOFFS
+                <span class="banner-year"></span>
+            </div>
+            <div class="banner-meta">
+                <span class="meta-round"></span>
+                <span class="meta-day hidden"></span>
+                <span class="meta-series"></span>
+                <span class="meta-elim hidden"></span>
+                <span class="meta-game7 hidden"></span>
+            </div>
+        </div>
+        <a class="banner-bracket-link" href="/bracket">Bracket</a>
+        <button class="banner-notify" type="button" title="Notify me on playoff OT" aria-label="Enable OT notifications">
+            <span class="bell-label">OT alerts</span>
+        </button>
+    </section>
     <main>
+        <section id="pinned-section" class="section pinned-section hidden">
+            <h2 class="section-heading section-heading-gold">Spotlight &middot; Game 7</h2>
+            <div id="pinned-games-section" class="games-grid"></div>
+        </section>
         <section id="live-section" class="section hidden">
             <h2 class="section-heading">Live</h2>
             <div id="live-games-section" class="games-grid"></div>