feat: always display country code in uppercase (model validates + oninput)

app/routers/web.py

@@ -221,7 +221,7 @@ def postcard_create(
         card_number=card_number,
         status=status,
         recipient_name=recipient_name or None,
-        country=country or None,
+        country=(country.strip().upper() if country else None),
         send_time=_parse_dt(send_time),
         arrival_time=_parse_dt(arrival_time),
         sender_name=sender_name or None,