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/Responses/MixApplicationResponse.cs

@@ -0,0 +1,27 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Mixes.Responses
+{
+    public record MixApplicationResponse(
+        Guid Id,
+        string Title,
+        string? Subtitle,
+        DateOnly GeneratedOn,
+        int ItemCount,
+        IReadOnlyList<MixItemApplicationResponse> Items)
+    {
+        public const int CoverItemCount = 4;
+
+        public static MixApplicationResponse FromEntity(
+            Mix mix,
+            int itemCount,
+            IReadOnlyList<MixItem> items) =>
+            new(
+                mix.Id,
+                mix.Title,
+                mix.Subtitle,
+                mix.GeneratedOn,
+                itemCount,
+                [.. items.Select(MixItemApplicationResponse.FromEntity)]);
+    }
+}