Fix avg_hr_day: remove dead averageHeartRate lookup; add max_hr_day from UDS export

garmin.get_stats() never returns averageHeartRate — avg_hr_day is only computable
from intraday HR which Garmin's API only serves for recent dates (~90-120 days).
The dead lookup gave false confidence that historical backfill would work.

Also populate max_hr_day from the Garmin export's UDS daily summaries (maxHeartRate
field is present for the full history), so historical max HR is available after
re-importing the export.

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

backend/app/services/garmin_connect_sync.py

@@ -524,7 +524,7 @@ def _parse_day(stats, sleep_data, hrv_data) -> dict:
 
     if stats:
         _set(row, "resting_hr",      stats.get("restingHeartRate"))
-        _set(row, "avg_hr_day",      stats.get("averageHeartRate"))
+        # averageHeartRate is absent from get_stats; avg_hr_day is computed below from intraday HR
         _set(row, "max_hr_day",      stats.get("maxHeartRate"))
         _set(row, "steps",           stats.get("totalSteps"))
         _set(row, "floors_climbed",  stats.get("floorsAscended"))