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/Hosts/Musify.Api/OpenApi/OpenApiExtensions.cs

@@ -1,5 +1,3 @@
-using Microsoft.AspNetCore.OpenApi;
-
 namespace Musify.Api.OpenApi;
 
 public static class OpenApiExtensions

backend/Hosts/Musify.Worker/Program.cs

@@ -1,6 +1,4 @@
 using Hangfire;
-using Mediator;
-using Microsoft.Extensions.DependencyInjection;
 using Musify.Application.Configuration;
 using Musify.Application.Services;
 using Musify.Infrastructure.Jobs;