Fixes: 1) Validate gateway presence in `process_transaction_changes` signal; 2) Add missing imports for `Type`, `create_object`, and `AbstractGateway`;
Extra: 1) Cleanup unused diff lines in migrations files; 2) Improve JSONField, FloatField, and PositiveIntegerField declarations by consolidating into single lines; 3) Remove redundant match-case logic in `signals.py`.