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/Contracts/IDatabase.cs

@@ -29,6 +29,10 @@ namespace Musify.Application.Contracts
 
         DbSet<PlayListHasTrack> PlayListHasTracks { get; }
 
+        DbSet<Mix> Mixes { get; }
+
+        DbSet<MixItem> MixItems { get; }
+
         DbSet<Upload> Uploads { get; }
 
         DbSet<UploadIntent> UploadIntents { get; }