First slice of the REST API (ersatztv#2). Idiomatic REST over existing MediatR handlers; design in docs/rest-api.md.
Foundation: NotFoundError : BaseError (Core) to split 404 from 422; ApiResults mapping helpers (Either/Option -> 201+Location / 200 / 204 / 404 / 422), additive (existing *ToActionResult untouched); ApiKeyAuthorizationFilter (optional X-Api-Key on mutating actions, gated by Api:WriteKey, no-op when unset) — independent of the IPTV JWT toggle so Jellyfin/Dispatcharr reads are unaffected, applied at controller-class level so every mutating action incl. ResetPlayout is covered fail-safe.
Channels: POST /api/channels (201+Location), PUT/{id} (200/404), DELETE/{id} (204/404), GET/{id} (200/404). Request DTOs -> existing commands; responses reuse ChannelViewModel. Ported page-only validations into handlers (ShowInEpg-when-disabled, external-logo-URL, Group NotEmpty) + FFmpegProfile/Watermark/Filler existence on update for create-parity. Fixed a latent 500: the .Filter(c>0) existence pattern threw TaskCanceledException on not-found; rewritten to AnyAsync.
Tests (NUnit, TZ=UTC): handler success/404/422, ApiResults mapping, API-key filter, controller status/Location/mapping, a controller-security regression net, and a create->read->delete EF integration test on a new in-memory SQLite harness. ErsatzTV.Tests 46/46, Architecture 5/5.
Refs #34
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
