fix: anchor Day N to each round's first game instead of lazy first sighting

The banner read "Day 1 of ~60" on day 2 of the playoffs because the old
anchor recorded whatever date we first polled a playoff game as Day 1.
Now round start dates come from /v1/schedule/playoff-series, so Day N
is authoritative and resets at each round boundary. Drops the noisy
"of ~60" denominator.

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

app/scheduler.py

@@ -4,7 +4,7 @@ import time
 import schedule
 
 from app.api import refresh_scores
-from app.playoff_cache import refresh_bracket
+from app.playoff_cache import refresh_bracket, refresh_round_start_dates
 from app.standings import refresh_standings
 
 logger = logging.getLogger(__name__)
@@ -14,8 +14,10 @@ def start_scheduler():
     schedule.every(600).seconds.do(refresh_standings)
     schedule.every(10).seconds.do(refresh_scores)
     schedule.every(3600).seconds.do(refresh_bracket)
+    schedule.every(21600).seconds.do(refresh_round_start_dates)
     # Populate the cache once at startup so the banner has data immediately.
     refresh_bracket()
+    refresh_round_start_dates()
     logger.info("Background scheduler started")
     while True:
         try: