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.Worker/AppSettings.json

@@ -26,6 +26,12 @@
     "ApplicationStorage": {
         "Bucket": ""
     },
+    "Mix": {
+        "ArtistMixCount": 3,
+        "ItemsPerMix": 12,
+        "SeedArtistCount": 3,
+        "YouTubeResultsPerSeed": 12
+    },
     "PlayList": {
         "Routes": {
             "ParentFolders": "PlayLists",

backend/Musify.Worker/Program.cs

@@ -1,10 +1,26 @@
+using Hangfire;
+using Mediator;
+using Microsoft.Extensions.DependencyInjection;
 using Musify.Application.Configuration;
+using Musify.Application.Services;
+using Musify.Infrastructure.Jobs;
 using Musify.Infrastructure.MassTransit;
 using Musify.Infrastructure.Persistence;
 using Musify.Infrastructure.Services;
 
 var builder = Host.CreateApplicationBuilder();
 
+// Registering Mediator pulls in every handler in Musify.Application, including ones
+// (e.g. CreatePlayList, CreateTrack) that depend on IEventBus, which the Worker never
+// registers because it only consumes events, it never publishes them. Only the handlers
+// this process actually dispatches (GenerateMixesForUserCommand) are ever constructed,
+// so the default eager validate-on-build check is too strict for this host.
+builder.ConfigureContainer(new DefaultServiceProviderFactory(new ServiceProviderOptions
+{
+    ValidateOnBuild = false,
+    ValidateScopes = true
+}));
+
 var configuration = builder.Configuration;
 
 var services = builder.Services;
@@ -12,6 +28,7 @@ var services = builder.Services;
 services.AddValidatedOptions<ApplicationStorageConfiguration>(configuration, ApplicationStorageConfiguration.SectionName);
 services.AddValidatedOptions<PlayListConfiguration>(configuration, PlayListConfiguration.SectionName);
 services.AddValidatedOptions<TrackConfiguration>(configuration, TrackConfiguration.SectionName);
+services.AddValidatedOptions<MixConfiguration>(configuration, MixConfiguration.SectionName);
 services.AddStorageService(configuration);
 services.AddAudioTranscoder(configuration);
 services.AddYouTubeDownloader(configuration);
@@ -20,7 +37,21 @@ services.AddPictureService(configuration);
 services.AddDatabase(configuration);
 services.AddMassTransitConsumers(configuration);
 services.AddUploadIntentJobs(configuration);
+services.AddMediator();
+services.AddScoped<YouTubeTrackProvisioner>();
+services.AddYouTubeMusicService(configuration);
+services.AddDailyMixGenerationJob(configuration);
 
 var host = builder.Build();
 
-host.Run();
+using (var scope = host.Services.CreateScope())
+{
+    var recurringJobManager = scope.ServiceProvider.GetRequiredService<IRecurringJobManager>();
+    recurringJobManager.AddOrUpdate<DailyMixGenerationJob>(
+        "daily-mix-generation",
+        job => job.RunAsync(CancellationToken.None),
+        Cron.Daily(),
+        new RecurringJobOptions { TimeZone = TimeZoneInfo.Utc });
+}
+
+host.Run();