Fix treadmill/indoor distance over-measure and clean up polluted PRs

- Garmin Connect sync now applies Garmins corrected summary distance/moving time,
  overriding the raw wrist-estimated FIT distance for treadmill/indoor runs
- Exclude indoor (no-GPS) runs from distance personal records (bogus fast splits)
- backfill_indoor_distances task: re-fetch corrected distance for historical indoor runs
- recompute_personal_records_all task: wipe and rebuild PRs from valid activities

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/services/garmin_connect_sync.py

@@ -181,7 +181,14 @@ def sync_activities(garmin, user_id: int, since: Optional[datetime],
         dest = dest_dir / f"{garmin_id}.fit"
         dest.write_bytes(fit_data)
 
-        process_activity_file.delay(str(dest), user_id, "fit", garmin_id)
+        # Pass Garmin's corrected summary so treadmill/indoor wrist-distance is
+        # overridden by the authoritative displayed distance/moving time.
+        summary = {
+            "distance": act.get("distance"),
+            "moving": act.get("movingDuration"),
+            "elapsed": act.get("elapsedDuration") or act.get("duration"),
+        }
+        process_activity_file.delay(str(dest), user_id, "fit", garmin_id, summary)
         queued += 1
 
         if status_callback and (queued % 5 == 0 or queued == total):