New endpoints:
- GET /analytics/gaps/{vertical} - Songs overdue for a play
- GET /analytics/velocity/{vertical} - Hot vs cooling songs
- GET /analytics/trends/{vertical} - Monthly/quarterly chart data
- GET /analytics/stats/{vertical} - Aggregate band statistics
- GET /analytics/bustouts/{vertical} - Songs returning after long gaps
- GET /analytics/debut-songs/{vertical} - Recently debuted songs
