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/MixItemApplicationResponse.cs

@@ -0,0 +1,27 @@
+using Musify.Domain.Entities;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.Mixes.Responses
+{
+    public record MixItemApplicationResponse(
+        MixItemSource Source,
+        Guid? TrackId,
+        string? VideoId,
+        string Title,
+        string? Artist,
+        string? ThumbnailUrl,
+        int DurationSeconds,
+        bool IsExplicit)
+    {
+        public static MixItemApplicationResponse FromEntity(MixItem item) =>
+            new(
+                item.Source,
+                item.TrackId,
+                item.VideoId,
+                item.Title ?? string.Empty,
+                item.Artist,
+                item.ThumbnailUrl,
+                item.DurationSeconds,
+                item.IsExplicit);
+    }
+}