Add backend test suite under backend/Tests/

Two xUnit projects, wired into Musify.slnx under a /Tests/ folder that
mirrors Core/ and Hosts/:

  Musify.Domain.Tests        Value-object logic (TrackAudio/TrackPictures'
                              IsProcessed/IsFailed/IsInProgress). Entities are
                              otherwise anemic EF models with no behavior to
                              test.
  Musify.Application.Tests   All ~45 Application handlers and services
                              (Albums, Tracks, PlayLists, Users, Mixes,
                              YouTube, plus TrackStreamIssuer,
                              UploadIntentValidator, YouTubeTrackProvisioner).

IDatabase exposes DbSet<T> directly, so handlers compose real LINQ queries
against it — that can't be expressed with a mock (no IQueryable provider
behind one). TestSupport/TestDatabase.cs is instead a real DbContext on a
private SQLite in-memory connection per test (fresh per test since xUnit
instantiates the test class per test method): fast, no external dependency,
and — unlike EF Core's InMemory provider — SQLite enforces real foreign keys
and supports transactions, which two handlers need. External dependencies
(IStorageService, IEventBus, IYouTubeMusicService, ...) are substituted with
NSubstitute.

TestSupport/TestEntities.cs and TestConfigurations.cs build fully-populated
domain entities and configuration POCOs with sensible defaults for every
required member, so no test method needs to fill in properties it doesn't
care about, and no builder ever hands back null.

Also, needed to make the Application layer itself testable without a real
Postgres or user-secrets:
- DatabaseDesignTimeFactory now falls back to the Database__ConnectionString
  env var, and DesignSettings.json is optional — unblocks the `migrate`
  one-shot container from deploy/compose.yml, which hit the same gap.
- Directory.Packages.props: xunit, xunit.runner.visualstudio, NSubstitute,
  Microsoft.EntityFrameworkCore.Sqlite (pinned to the same 10.0.8 as the rest
  of EF Core), Microsoft.NET.Test.Sdk, coverlet.collector. Also pins
  SQLitePCLRaw.bundle_e_sqlite3 to 2.1.13 — EFCore.Sqlite 10.0.8 pulls 2.1.11
  transitively, which has a known high-severity advisory (GHSA-2m69-gcr7-jv3q).
- backend/.dockerignore excludes Tests/: no application Dockerfile needs it
  in its build context.

162 tests, all green: `dotnet test backend/Musify.slnx`.

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

@@ -0,0 +1,86 @@
+using ErrorOr;
+using Musify.Application.Configuration;
+using Musify.Application.Contracts;
+using Musify.Application.PlayLists;
+using Musify.Application.Services;
+using Musify.Application.Tests.TestSupport;
+using NSubstitute;
+using Xunit;
+
+namespace Musify.Application.Tests.PlayLists;
+
+public sealed class CreatePlayListCommandHandlerTests : HandlerTestBase
+{
+    private readonly IEventBus eventBus = Substitute.For<IEventBus>();
+    private readonly IStorageService storageService = Substitute.For<IStorageService>();
+
+    private CreatePlayListCommandHandler CreateHandler() => new(
+        eventBus,
+        Database,
+        new UploadIntentValidator(Database, storageService),
+        NoOpLogger<CreatePlayListCommandHandler>(),
+        TestConfigurations.Storage(),
+        TestConfigurations.PlayList(),
+        TestConfigurations.UploadIntent());
+
+    [Fact]
+    public async Task Handle_NoPictureIntent_CreatesPlayListWithPresetPictures()
+    {
+        var user = TestEntities.User();
+        await SeedAsync(user);
+
+        var command = new CreatePlayListCommand(user.Id, "My Playlist", "A description", PictureIntentId: null);
+
+        var result = await CreateHandler().Handle(command, CancellationToken.None);
+
+        Assert.False(result.IsError);
+        Assert.Equal("My Playlist", result.Value.Name);
+        Assert.Equal("PresetSmallPicture.webp", result.Value.SmallImageKeyName);
+        await eventBus.DidNotReceive().PublishAsync(Arg.Any<Musify.Application.Events.CreatePlayListResourcesEvent>(), Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task Handle_WithValidPictureIntent_PublishesResourcesEvent()
+    {
+        storageService
+            .HeadObjectAsync(Arg.Any<string>(), Arg.Any<string>(), Arg.Any<CancellationToken>())
+            .Returns(new ObjectMetaData("image/webp", 1024));
+
+        var user = TestEntities.User();
+        var intent = TestEntities.UploadIntent(user.Id, Musify.Domain.ValueObjects.UploadIntentPurpose.PlayListPicture, objectName: "cover.webp");
+        await SeedAsync(user, intent);
+
+        var command = new CreatePlayListCommand(user.Id, "My Playlist", null, intent.Id);
+
+        var result = await CreateHandler().Handle(command, CancellationToken.None);
+
+        Assert.False(result.IsError);
+        var stored = await Database.PlayLists.FindAsync(result.Value.Id);
+        Assert.NotNull(stored);
+        Assert.Equal("cover.webp", stored.Pictures.OriginalName);
+        await eventBus.Received(1).PublishAsync(Arg.Any<Musify.Application.Events.CreatePlayListResourcesEvent>(), Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task Handle_UserMissing_ReturnsNotFound()
+    {
+        var command = new CreatePlayListCommand(404, "Orphan Playlist", null, null);
+
+        var result = await CreateHandler().Handle(command, CancellationToken.None);
+
+        Assert.Equal(ErrorType.NotFound, result.FirstError.Type);
+    }
+
+    [Fact]
+    public async Task Handle_PictureIntentNotFound_ReturnsError()
+    {
+        var user = TestEntities.User();
+        await SeedAsync(user);
+
+        var command = new CreatePlayListCommand(user.Id, "My Playlist", null, Guid.NewGuid());
+
+        var result = await CreateHandler().Handle(command, CancellationToken.None);
+
+        Assert.True(result.IsError);
+    }
+}