fix(235): make QueueShowScanResult switch total (explicit NotFound + throwing fallback)

Codex-review Low: NotFound previously reached the catch-all by coincidence; a
future enum value would silently 404. Explicit arm + UnreachableException fallback
so an unmapped outcome fails loudly rather than mis-mapping to 404.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/Controllers/Api/LibrariesController.cs

@@ -1,3 +1,4 @@
+using System.Diagnostics;
 using ErsatzTV.Application.Libraries;
 using ErsatzTV.Core.Api.Libraries;
 using ErsatzTV.Core.Interfaces.Repositories;
@@ -93,7 +94,8 @@ public class LibrariesController(ITelevisionRepository televisionRepository, IMe
                         Title = "Unable to scan show",
                         Detail = $"The scan for show {request.ShowId} in library {id} could not be completed."
                     }),
-                _ => ApiResults.NotFoundProblem($"Library {id} does not exist.")
+                QueueShowScanResult.NotFound => ApiResults.NotFoundProblem($"Library {id} does not exist."),
+                _ => throw new UnreachableException($"Unmapped QueueShowScanResult: {result}")
             };
         }