Fix API spec to match actual response types

Discovered while building the ePOS staff app against the staging API
that several schema definitions don't match the real responses:

- room.id: Changed from string to integer. The API returns numeric
  IDs (e.g. 1, 2, 3), not string codes.
- customer.regular_bookings_count: Can return boolean `false` instead
  of integer `0` when the customer has no regular bookings. Updated
  to oneOf [integer, boolean].
- short_group: Added `number` field (nullable integer, used in
  customer band responses) and `booking_count` field. The existing
  `number_of_members` is retained for full group responses.

https://claude.ai/code/session_01AUVAYoQ6vtiGeSvWiGsfpK

Author: claude