Enhance logging in RefreshShowEpisodesMetadataCommandHandler and TmdbMetadataProvider for image retrieval failures

Updated the RefreshShowEpisodesMetadataCommandHandler to include logging for scenarios where episode still images are not retrieved successfully. Introduced warning logs for missing still URLs and download failures, improving traceability. Additionally, modified the TmdbMetadataProvider to implement a fallback mechanism for episode still images, ensuring that appropriate imagery is displayed even when localized images are unavailable. Updated related tests to verify the correct behavior of these logging enhancements and fallback logic.

backend/tests/TeleWave.Application.Tests/Metadata/RefreshEpisodesTests.cs

@@ -1,4 +1,5 @@
 using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging.Abstractions;
 using NSubstitute;
 using TeleWave.Application.Common.Interfaces;
 using TeleWave.Application.Metadata;
@@ -39,7 +40,8 @@ public class RefreshEpisodesTests
             db,
             resolver,
             downloader ?? Substitute.For<IImageDownloader>(),
-            store ?? Substitute.For<IImageStore>()
+            store ?? Substitute.For<IImageStore>(),
+            NullLogger<RefreshShowEpisodesMetadataCommandHandler>.Instance
         );
 
     /// <summary>Сериал с привязкой к источнику и одной серией на готовом ассете.</summary>

backend/tests/TeleWave.Application.Tests/Metadata/TmdbMetadataProviderTests.cs

@@ -146,6 +146,26 @@ public class TmdbMetadataProviderTests
         Assert.EndsWith("/neutral.jpg", show?.PosterUrl);
     }
 
+    /// <summary>Кадр серии — та же история, что и с постером шоу: под русский язык его часто нет.</summary>
+    [Fact]
+    public async Task GetEpisode_WithoutLocalizedStill_FallsBackToNeutralOne()
+    {
+        var http = new FakeHttpClientFactory(url =>
+            FakeHttpClientFactory.Ok(
+                url.Contains("language=", StringComparison.Ordinal)
+                    ? """{"name":"Серия"}"""
+                    : """{"name":"Серия","still_path":"/neutral.jpg"}"""
+            )
+        );
+
+        var episode = await Provider(http).GetEpisodeAsync("9", 2, 5, CancellationToken.None);
+
+        Assert.Equal(2, http.Urls.Count);
+        Assert.DoesNotContain("language=", http.LastUrl);
+        Assert.Contains("/tv/9/season/2/episode/5", http.LastUrl);
+        Assert.EndsWith("/neutral.jpg", episode?.StillUrl);
+    }
+
     [Fact]
     public async Task GetShow_Movie_FallsBackToAnyReleaseCertification()
     {