Refactor ShowAudience handling across the application to align with MPAA rating system. Update ShowAudience enum to reflect new ratings (G, PG, PG-13, R, NC-17) and adjust related data models, API types, and frontend components to support nullable audience values. Enhance metadata handling to incorporate content ratings from external sources, ensuring proper audience filtering in scheduling logic. Update documentation to clarify changes in audience categorization and its implications for content management.

backend/src/TeleWave.Application/Library/Collections/CollectionDtos.cs

@@ -20,7 +20,7 @@ public sealed record CollectionItemDto(
     int Position,
     string ShowName,
     ShowKind ShowKind,
-    ShowAudience ShowAudience,
+    ShowAudience? ShowAudience,
     int EpisodeCount,
     int? Year,
     Guid? PosterImageId

backend/src/TeleWave.Application/Library/Collections/GetCollection/GetCollectionQueryHandler.cs

@@ -46,7 +46,7 @@ public sealed class GetCollectionQueryHandler(IAppDbContext dbContext)
                     i.Position,
                     show?.Name ?? "—",
                     show?.Kind ?? default,
-                    show?.Audience ?? default,
+                    show?.Audience,
                     show?.EpisodeCount ?? 0,
                     show?.Year,
                     show?.PosterImageId