UpdateChannelHandler.Validate never checked incoming graphicsElementIds against
GraphicsElements, so PUT /api/v1/channels/{id} with a non-existent id hit
FK_ChannelGraphicsElement_GraphicsElement_GraphicsElementId at SaveChangesAsync
and surfaced as an unhandled 500. Add GraphicsElementIdsMustExist, following the
existing FFmpegProfileMustExist/WatermarkMustExist/FillerPresetMustExist shape,
so an unknown id now returns 422 for parity with every other FK field on this
full-replace DTO.
GetAllGraphicsElementsForApiHandler and GraphicsElementSeeder.GetBuiltInElementId
keyed builtIn off Path.GetFileName(e.Path) == OnNowNextFileName -- folder-agnostic,
so a user element named exactly on-now-next.yml in any other template folder would
also report builtIn:true. Both now compare against
GraphicsElementDefaults.OnNowNextSeededPath, the full path the seeder actually
writes to.
Follow-up from the #74 whole-branch review (2026-07-22), deferred as
data-safe/not SPA-reachable.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015QqCpYFsKgnAnx6jVwrKiV