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",

backend/Musify.Api/Endpoints/MixEndpoints.cs

@@ -0,0 +1,51 @@
+using Mediator;
+using Musify.Api.Authentication;
+using Musify.Api.Extensions;
+using Musify.Application.Mixes;
+using Musify.Application.Mixes.Responses;
+
+namespace Musify.Api.Endpoints;
+
+public static class MixEndpoints
+{
+    public static IEndpointRouteBuilder MapMixEndpoints(this IEndpointRouteBuilder app)
+    {
+        var group = app.MapGroup("/mixes")
+            .WithTags("Mixes")
+            .RequireAuthorization();
+
+        group.MapGet("/", GetMixes)
+            .WithName("GetMixes")
+            .WithSummary("Get The Server-Generated Mixes Of The Current User.")
+            .Produces<IReadOnlyList<MixApplicationResponse>>()
+            .Produces(StatusCodes.Status401Unauthorized);
+
+        group.MapGet("/{id}", GetMixById)
+            .WithName("GetMixById")
+            .WithSummary("Get A Mix With All Of Its Songs.")
+            .Produces<MixApplicationResponse>()
+            .Produces(StatusCodes.Status401Unauthorized)
+            .Produces(StatusCodes.Status404NotFound);
+
+        return app;
+    }
+
+    private static async Task<IResult> GetMixes(
+        IMediator mediator,
+        CurrentUser currentUser,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new GetMixesByUserIdQuery(currentUser.RequiredId), cancellationToken);
+        return result.ToHttpResult();
+    }
+
+    private static async Task<IResult> GetMixById(
+        IMediator mediator,
+        CurrentUser currentUser,
+        Guid id,
+        CancellationToken cancellationToken)
+    {
+        var result = await mediator.Send(new GetMixByIdQuery(currentUser.RequiredId, id), cancellationToken);
+        return result.ToHttpResult();
+    }
+}

backend/Musify.Api/Program.cs

@@ -39,6 +39,7 @@ app.UseAuthorization();
 
 app
     .MapAlbumEndpoints()
+    .MapMixEndpoints()
     .MapPlayListEndpoints()
     .MapTrackEndpoints()
     .MapUserEndpoints()