feat: recommend server-generated mixes from Musify and YouTube Music

Mixes are their own concept, not playlists: they live in their own
Mixes/MixItems tables, cannot be edited and never show up among the
user's playlists. Each item is either a Musify track or a YouTube Music
reference, so YouTube songs stream directly instead of being downloaded.

A daily Hangfire job seeds the mixes from the listening history: one
discovery mix, one daily mix and one per top artist.

The per-user YouTube filler query was already in the working tree and
rides along here because it shares files with this change.

backend/Musify.Application/Mixes/GetMixById.cs

@@ -0,0 +1,30 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Mixes.Responses;
+
+namespace Musify.Application.Mixes
+{
+    public record GetMixByIdQuery(long UserId, Guid MixId)
+        : IQuery<ErrorOr<MixApplicationResponse>>;
+
+    public class GetMixByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetMixByIdQuery, ErrorOr<MixApplicationResponse>>
+    {
+        public async ValueTask<ErrorOr<MixApplicationResponse>> Handle(GetMixByIdQuery request, CancellationToken cancellationToken)
+        {
+            var mix = await database.Mixes
+                .AsNoTracking()
+                .Include(mix => mix.Items)
+                .SingleOrDefaultAsync(mix => mix.Id == request.MixId && mix.UserId == request.UserId, cancellationToken);
+
+            if (mix is null)
+                return Error.NotFound(description: $"Mix {request.MixId} was not found");
+
+            var items = mix.Items.OrderBy(item => item.Position).ToList();
+
+            return MixApplicationResponse.FromEntity(mix, items.Count, items);
+        }
+    }
+}