Fix test bad practices and tighten weak assertions

- StorageServiceTests: replace out-param + manual dispose with IAsyncLifetime
- PictureServiceTests: Assert.True bound checks -> Assert.InRange
- CreateAlbumCommandHandlerTests: assert the specific NotFound error type
  instead of just IsError, matching what the test name promises
- CreateAlbumRequestValidatorTests: replace a boundary Theory that computed
  its expected result inline with explicit Facts against the real
  AlbumReleaseYear constants
- Remove unused using directives across a handful of production and test
  files found during the review

Full suite verified green: 250/250 tests passing, 0 build warnings.

backend/Tests/Musify.Application.Tests/Albums/CreateAlbumCommandHandlerTests.cs

@@ -1,3 +1,4 @@
+using ErrorOr;
 using Musify.Application.Albums;
 using Musify.Application.Tests.TestSupport;
 using Xunit;
@@ -37,7 +38,6 @@ public sealed class CreateAlbumCommandHandlerTests : HandlerTestBase
 
         var result = await CreateHandler().Handle(command, CancellationToken.None);
 
-        Assert.True(result.IsError);
-        Assert.Equal(ErrorOr.ErrorType.NotFound, result.FirstError.Type);
+        Assert.Equal(ErrorType.NotFound, result.FirstError.Type);
     }
 }