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/Configuration/MixConfiguration.cs

@@ -0,0 +1,21 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace Musify.Application.Configuration
+{
+    public class MixConfiguration
+    {
+        public const string SectionName = "Mix";
+
+        [Range(1, 20)]
+        public int ArtistMixCount { get; set; } = 3;
+
+        [Range(4, 100)]
+        public int ItemsPerMix { get; set; } = 12;
+
+        [Range(1, 10)]
+        public int SeedArtistCount { get; set; } = 3;
+
+        [Range(1, 100)]
+        public int YouTubeResultsPerSeed { get; set; } = 12;
+    }
+}