Live E2E found PUT/DELETE on a missing filler preset or watermark returned 422
instead of 404. Root cause: the must-exist checks fed a NotFoundError through
LanguageExt Validation, whose aggregation to Either flattens the BaseError
subtype, so ApiResults.ToErrorResult never saw a NotFoundError. The precedent
handlers (DeleteProgramSchedule, DeleteFFmpegProfile, UpdateProgramSchedule)
avoid this by resolving must-exist as an Option and returning the NotFoundError
directly as an Either Left via Option.Match — restructured the four
filler/watermark Update/Delete handlers to that pattern (remaining name
validation still 422s). No Blazor behavior change (it only reads error.Value).
Verified live: PUT/DELETE missing → 404, GET missing → 404, empty-name POST →
422; happy-path CRUD unchanged.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
timothyandClaude Fable 5