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.Api/AppSettings.json

@@ -57,6 +57,12 @@
         "ThumbnailSize": 240,
         "ArtworkSize": 544
     },
+    "Mix": {
+        "ArtistMixCount": 3,
+        "ItemsPerMix": 12,
+        "SeedArtistCount": 3,
+        "YouTubeResultsPerSeed": 12
+    },
     "PlayList": {
         "Routes": {
             "UploadsFolder": "uploads",