GetById/Create/Update return ChannelResponseModel via new GetChannelByIdForApi
read-side query; POST /api/channels/{id:int}/playout/reset (new
GetPlayoutIdByChannelId; by-number kept for HlsSessionWorker broadcast).
Refs #288#197
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>