[API] Expose changelog history

[cka-y]

Describe the problem

There are no API endpoints to query a feed's change history or retrieve a changelog artifact.

Proposed solution

1. GET /v1/gtfs_feeds/{id}/changelog

• Queries gtfs_dataset_changelog by feed_id
• Returns a paginated, chronologically sorted list of change events
• Each entry includes: id, generated_at, previous_dataset_id, current_dataset_id, changelog_url
• Filter params: start_date, end_date

2. GET /v1/gtfs_feeds/{id}/datasets/{dataset_id}/changelog

• Returns the single gtfs_dataset_changelog entry where current_dataset_id = dataset_id
• Convenience endpoint for callers who have a dataset ID and want its diff