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.Domain/ValueObjects/MixItemSource.cs

@@ -0,0 +1,8 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum MixItemSource
+    {
+        Musify = 0,
+        YouTube = 1
+    }
+}