fix: clear arrival_time and disable field when status is sent

app/routers/web.py

@@ -354,7 +354,7 @@ def postcard_update(
     pc.recipient_name = recipient_name or None
     pc.country = country or None
     pc.send_time = _parse_dt(send_time)
-    pc.arrival_time = _parse_dt(arrival_time)
+    pc.arrival_time = None if status == "sent" else _parse_dt(arrival_time)
     pc.sender_name = sender_name or None
     pc.receive_time = _parse_dt(receive_time)
     db.commit()