Fix sync_lookback_days actually controlling the sync window

Activities: lookback_days was ignored once last_sync_at was set (since
always took priority). Now lookback_days always sets the window; -1 is
all-time on first sync then incremental.

Wellness: lookback_days was never passed to sync_wellness at all —
hardcoded 90-day cap regardless of settings. Fixed by adding lookback_days
param and wiring it through from the Celery task.

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

backend/app/workers/tasks.py

@@ -507,7 +507,10 @@ def sync_garmin_connect_user(user_id: int):
 
         if cfg.sync_wellness:
             try:
-                wellness_days = sync_wellness(garmin, user_id, cfg.last_sync_at, db)
+                wellness_days = sync_wellness(
+                    garmin, user_id, cfg.last_sync_at, db,
+                    lookback_days=cfg.sync_lookback_days if cfg.sync_lookback_days is not None else 90,
+                )
             except Exception as exc:
                 errors.append(f"wellness: {exc}")