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/Core/Musify.Infrastructure/Persistence/DatabaseDependencyInjection.cs

@@ -1,4 +1,3 @@
-using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Options;

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260705123932_AddTrackProcessingStateSaga.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260705125356_AddPlayListProcessingStateSaga.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260713234847_AddListeningHistory.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260716214752_AddArtistsAndTrackOwnership.cs

@@ -1,4 +1,3 @@
-using System;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260717172507_SplitTrackIntoLocalAndExternal.cs

@@ -1,4 +1,3 @@
-using System;
 using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260720180747_AddAlbums.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260721155102_AddMixes.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260721162102_DropMixGeneratedOn.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable
 

backend/Core/Musify.Infrastructure/Persistence/Migrations/20260724181012_AddExternalAlbums.cs

@@ -1,5 +1,4 @@
-using System;
-using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Migrations;
 
 #nullable disable