fix: pass user_id=None in scheduler sync log to avoid FK constraint

sync_wa_log.user_id has a FK to users; passing 0 fails since no user
with id=0 exists. Scheduler runs without a user context, so use NULL.

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

app/services/scheduler.py

@@ -168,7 +168,7 @@ async def sync_recientes(ventana_min: int = 2) -> dict:
                                  modo="upsert", examenes_map=examenes_map)
 
     if resultado["total"] > 0:
-        guardar_sync_log(resultado, 0, origen="scheduler", modo="upsert")
+        guardar_sync_log(resultado, None, origen="scheduler", modo="upsert")
 
     return {
         "ok":          True,