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.Infrastructure/Services/ServicesDependencyInjection.cs

@@ -1,4 +1,6 @@
 using Amazon.S3;
+using Hangfire;
+using Hangfire.PostgreSql;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Options;
@@ -60,6 +62,21 @@ namespace Musify.Infrastructure.Services
             return serviceDescriptors;
         }
 
+        public static IServiceCollection AddDailyMixGenerationJob(this IServiceCollection serviceDescriptors, IConfiguration configuration)
+        {
+            serviceDescriptors.AddHangfire((serviceProvider, hangfireConfiguration) =>
+            {
+                var databaseConfiguration = serviceProvider.GetRequiredService<DatabaseConfiguration>();
+                hangfireConfiguration.UsePostgreSqlStorage(options =>
+                    options.UseNpgsqlConnection(databaseConfiguration.ConnectionString));
+            });
+
+            serviceDescriptors.AddHangfireServer();
+            serviceDescriptors.AddScoped<DailyMixGenerationJob>();
+
+            return serviceDescriptors;
+        }
+
         public static IServiceCollection AddStreamTicketService(this IServiceCollection serviceDescriptors, IConfiguration configuration)
         {
             serviceDescriptors