Add configurable sync_lookback_days for Garmin Connect

Users can now set how many days back the first sync fetches. -1 syncs all
history back to 2010; any positive value sets a rolling window. Values
over 365 show a rate-limit warning in the UI. The default remains 30 days.

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

backend/app/workers/tasks.py

@@ -499,7 +499,8 @@ def sync_garmin_connect_user(user_id: int):
         if cfg.sync_activities:
             try:
                 activities_queued = sync_activities(
-                    garmin, user_id, cfg.last_sync_at, db, settings.file_store_path
+                    garmin, user_id, cfg.last_sync_at, db, settings.file_store_path,
+                    lookback_days=cfg.sync_lookback_days if cfg.sync_lookback_days is not None else 30,
                 )
             except Exception as exc:
                 errors.append(f"activities: {exc}")