fix(#193): review follow-ups — drop Playlist option, non-tracking existence check

Adversarial review follow-ups on the #193 rerun selection validation:
- RerunCollectionEditor.razor: remove the CollectionType.Playlist
  dropdown option — RerunCollection has no PlaylistId column, so the
  option previously persisted a broken row and now fails validation.
- RerunCollectionSelectionValidation: replace FindAsync with a
  non-tracking AnyAsync(EF.Property<int>("Id") == id) existence check
  so nothing is materialized/tracked into the save-path context.
- Tests: pin the deliberate 422-not-404 choice
  (ShouldNotBeOfType<NotFoundError>) and cover the unsupported-type
  default arm (Playlist -> "Unsupported collection type"), which was
  reachable via the Blazor editor.

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

ErsatzTV/Pages/RerunCollectionEditor.razor

@@ -36,7 +36,6 @@
                     <MudSelectItem Value="CollectionType.Artist">Artist</MudSelectItem>
                     <MudSelectItem Value="CollectionType.MultiCollection">Multi Collection</MudSelectItem>
                     <MudSelectItem Value="CollectionType.SmartCollection">Smart Collection</MudSelectItem>
-                    <MudSelectItem Value="CollectionType.Playlist">Playlist</MudSelectItem>
                 </MudSelect>
             </MudStack>
             @if (_model.CollectionType == CollectionType.Collection)