feat(backend): add playlist cover endpoint and relax description validation

Adds GET /playlists/{id}/cover (mirrors the track cover endpoint) so
uploaded playlist pictures can actually be served. Also drops the
NotEmpty rule on playlist description, which rejected the blank
description the frontend treats as optional; CreatePlayList now falls
back to the domain's default text instead.

backend/Musify.Application/Musify.Application.csproj

@@ -13,6 +13,7 @@
     <PackageReference Include="Microsoft.Extensions.Configuration.Binder" />
     <PackageReference Include="Microsoft.Extensions.Options.ConfigurationExtensions" />
     <PackageReference Include="Microsoft.Extensions.Options.DataAnnotations" />
+    <PackageReference Include="MimeMapping" />
     <PackageReference Include="X.PagedList" />
     <PackageReference Include="X.PagedList.EF" />
   </ItemGroup>

backend/Musify.Application/PlayLists/GetPlayListCover.cs

@@ -0,0 +1,49 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using MimeMapping;
+using Musify.Application.Configuration;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists
+{
+    public record GetPlayListCoverQuery(Guid PlayListId, string Size)
+        : IQuery<ErrorOr<PlayListCoverLocation>>;
+
+    public record PlayListCoverLocation(string Bucket, string Key, string ContentType);
+
+    public class GetPlayListCoverQueryHandler(
+        IDatabase database,
+        ApplicationStorageConfiguration storageConfiguration,
+        PlayListConfiguration playListConfiguration)
+        : IQueryHandler<GetPlayListCoverQuery, ErrorOr<PlayListCoverLocation>>
+    {
+        public async ValueTask<ErrorOr<PlayListCoverLocation>> Handle(GetPlayListCoverQuery request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists.AsNoTracking()
+                .Where(p => p.Id == request.PlayListId)
+                .Select(p => new { p.SmallPictureName, p.MediumPictureName, p.LargePictureName })
+                .SingleOrDefaultAsync(cancellationToken);
+
+            if (playList == null)
+                return Error.NotFound();
+
+            var routes = playListConfiguration.Routes;
+            var (name, key, presetName) = request.Size.ToLowerInvariant() switch
+            {
+                "small" => (playList.SmallPictureName, BuildKey(routes.BuildSmallPicturePath, playList.SmallPictureName), routes.PresetSmallPicture),
+                "large" => (playList.LargePictureName, BuildKey(routes.BuildLargePicturePath, playList.LargePictureName), routes.PresetLargePicture),
+                _ => (playList.MediumPictureName, BuildKey(routes.BuildMediumPicturePath, playList.MediumPictureName), routes.PresetMediumPicture)
+            };
+
+            if (string.IsNullOrEmpty(name) || string.IsNullOrEmpty(key) || name == presetName)
+                return Error.NotFound();
+
+            var contentType = MimeUtility.GetMimeMapping(name);
+            return new PlayListCoverLocation(storageConfiguration.Bucket, key, contentType);
+        }
+
+        private static string? BuildKey(Func<string, string> builder, string? name)
+            => string.IsNullOrEmpty(name) ? null : builder(name);
+    }
+}