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/scheduler.py

@@ -4,6 +4,7 @@ import time
 import schedule
 
 from app.api import refresh_scores
+from app.playoff_cache import refresh_bracket
 from app.standings import refresh_standings
 
 logger = logging.getLogger(__name__)
@@ -12,6 +13,9 @@ logger = logging.getLogger(__name__)
 def start_scheduler():
     schedule.every(600).seconds.do(refresh_standings)
     schedule.every(10).seconds.do(refresh_scores)
+    schedule.every(3600).seconds.do(refresh_bracket)
+    # Populate the cache once at startup so the banner has data immediately.
+    refresh_bracket()
     logger.info("Background scheduler started")
     while True:
         try: