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/PlayLists/AddYouTubeTrackToPlayListCommandHandlerTests.cs

@@ -5,7 +5,6 @@ using Musify.Application.PlayLists;
 using Musify.Application.Services;
 using Musify.Application.Tests.TestSupport;
 using Musify.Domain.Entities;
-using Musify.Domain.ValueObjects;
 using NSubstitute;
 using Xunit;
 

backend/Tests/Musify.Application.Tests/PlayLists/CreatePlayListCommandHandlerTests.cs

@@ -1,5 +1,4 @@
 using ErrorOr;
-using Musify.Application.Configuration;
 using Musify.Application.Contracts;
 using Musify.Application.PlayLists;
 using Musify.Application.Services;