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.Infrastructure.Tests/Services/PictureServiceTests.cs

@@ -27,9 +27,9 @@ public sealed class PictureServiceTests
         await using var resized = await CreateService().ResizePictureAsWebpAsync(source, 200, 200, CancellationToken.None);
 
         using var result = await Image.LoadAsync(resized);
-        Assert.True(result.Width <= 200);
-        Assert.True(result.Height <= 200);
-        Assert.True(result.Width == 200 || result.Height == 200);
+        Assert.InRange(result.Width, 1, 200);
+        Assert.InRange(result.Height, 1, 200);
+        Assert.True(result.Width == 200 || result.Height == 200, "expected the longer side to land exactly on the target box");
     }
 
     [Fact]