Fixed registered field having wrong timezone

This happened with at least Postgres, which took it's server's timezone. It's complicated. Let's just set it from our side instead where we have some control.

tests_integration/tests/vendor/test_add.py

@@ -1,5 +1,7 @@
 """Integration tests for the Vendor API endpoint."""
 
+from datetime import datetime, timezone
+
 import httpx
 
 URL = "http://spoolman:8000"
@@ -25,6 +27,10 @@ def test_add_vendor():
         "comment": comment,
     }
 
+    # Verify that registered happened almost now (within 1 minute)
+    diff = abs((datetime.now(tz=timezone.utc) - datetime.fromisoformat(vendor["registered"])).total_seconds())
+    assert diff < 60
+
     # Clean up
     httpx.delete(f"{URL}/api/v1/vendor/{vendor['id']}").raise_for_status()