refactor: migrate api_client.py prints to structured logging

Replace all print() calls with logger.info/logger.error using
lazy % formatting. Add test verifying error path emits ERROR log.

U2 of P2-001 (replace print with logging)

tests/test_api_client.py

@@ -0,0 +1,19 @@
+import logging
+from unittest.mock import patch
+
+import pytest
+
+from api_client import TweedeKamerAPI
+
+
+class TestTweedeKamerAPILogging:
+    def test_get_motions_error_logged(self, caplog):
+        api = TweedeKamerAPI()
+        caplog.set_level(logging.ERROR, logger="api_client")
+
+        with patch.object(api, "_get_voting_records", side_effect=Exception("network down")):
+            result = api.get_motions()
+
+        assert result == []
+        assert any("network down" in rec.message for rec in caplog.records)
+        assert any(rec.levelno == logging.ERROR for rec in caplog.records)