fix(kattis): harden reactive re-auth trigger on submit

Problem: stale cookie detection only matched the exact text
`"Request validation failed"`, missing cases where Kattis returns
a 400 or 403 status instead.

Solution: also trigger re-login when `r.status_code in (400, 403)`.

scrapers/kattis.py

@@ -366,7 +366,7 @@ class KattisScraper(BaseScraper):
             except Exception as e:
                 return self._submit_error(f"Submit request failed: {e}")
 
-            if r.text == "Request validation failed":
+            if r.status_code in (400, 403) or r.text == "Request validation failed":
                 _COOKIE_PATH.unlink(missing_ok=True)
                 print(json.dumps({"status": "logging_in"}), flush=True)
                 ok = await _do_kattis_login(client, username, password)