Enhance ShowMetadataApplier with logging for poster retrieval failures

Updated the ShowMetadataApplier class to include logging for scenarios where show posters are not retrieved successfully. Introduced logging statements to capture warnings when poster URLs are empty or when downloads fail, improving traceability and debugging capabilities. Additionally, updated related tests to ensure proper logging behavior during poster processing.

backend/tests/TeleWave.Application.Tests/Library/AddShowsToCollectionTests.cs

@@ -1,4 +1,5 @@
-using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging.Abstractions;
 using NSubstitute;
 using TeleWave.Application.Common.Interfaces;
 using TeleWave.Application.Library.Collections.AddShowsToCollection;
@@ -98,7 +99,8 @@ public class AddShowsToCollectionTests
                 resolver,
                 Substitute.For<IImageDownloader>(),
                 Substitute.For<IImageStore>(),
-                new GenreMatcher(db)
+                new GenreMatcher(db),
+                NullLogger<ShowMetadataApplier>.Instance
             )
         ).Handle(new EnrichShowsMetadataCommand([Guid.NewGuid()], "tmdb"), CancellationToken.None);
 
@@ -130,7 +132,8 @@ public class AddShowsToCollectionTests
                 resolver,
                 Substitute.For<IImageDownloader>(),
                 Substitute.For<IImageStore>(),
-                new GenreMatcher(db)
+                new GenreMatcher(db),
+                NullLogger<ShowMetadataApplier>.Instance
             )
         ).Handle(new EnrichShowsMetadataCommand([clip.Id], "tmdb"), CancellationToken.None);