feat(api): filler preset + watermark editor CRUD; ffmpeg profile round-trip (#159)

Backend plumbing for the SPA transcoding editors (#143 blocker):

- FFmpeg profile response DTO round-trip: add NormalizeAudio, NormalizeVideo,
  PadMode, TargetLoudness, NormalizeColors, ResolutionId (keep Resolution name
  for compat); DeinterlaceVideo now bool (was bool?). #nullable enable on the DTO.
- New GET /api/ffmpeg/hardware-acceleration-kinds wrapping
  GetSupportedHardwareAccelerationKinds (returns enum names as strings).
- Filler preset CRUD: GET-by-id/POST/PUT/DELETE on FillerPresetController with
  full request/response DTOs; new FillerPresetFullResponseModel +
  GetFillerPresetByIdForApi. CreateFillerPreset now returns the new id
  (CreateFillerPresetResult) to match sibling Create commands and enable a
  201+Location; updated the one Blazor call site.
- Watermark CRUD: GET-by-id/POST/PUT/DELETE on WatermarkController with full
  DTOs (path+contentType, all 15 fields); new WatermarkFullResponseModel +
  GetWatermarkByIdForApi.
- Startup: register WatermarkLocation/WatermarkSize as OpenAPI string enums
  (they live in ErsatzTV.FFmpeg.State and were documented as ints, mismatching
  the Newtonsoft StringEnumConverter runtime serialization).
- Tests: full CRUD controller tests for filler + watermark; contract-test
  entries (404/401/422) for the new mutating endpoints; regenerated v1.json.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/Pages/FillerPresetEditor.razor

@@ -399,9 +399,11 @@
         ValidationResult result = await _validator.ValidateAsync(_model, _cts.Token);
         if (result.IsValid)
         {
-            IRequest<Either<BaseError, Unit>> request = IsEdit ? _model.ToEdit() : _model.ToUpdate();
-
-            Seq<BaseError> errorMessage = (await Mediator.Send(request, _cts.Token)).LeftToSeq();
+            // Create/Update return different result types (Update -> Unit, Create -> CreateFillerPresetResult),
+            // so send each on its own branch and reduce to the shared Left error sequence.
+            Seq<BaseError> errorMessage = IsEdit
+                ? (await Mediator.Send(_model.ToEdit(), _cts.Token)).LeftToSeq()
+                : (await Mediator.Send(_model.ToUpdate(), _cts.Token)).LeftToSeq();
 
             errorMessage.HeadOrNone().Match(
                 error =>