Add track publication, album types, optional track pictures and batch upload URLs

Tracks
- Track.IsPublished (migration marks existing tracks as published). New
  tracks are created unpublished.
- PUT /tracks/{id}/published and PUT /tracks/published change the flag;
  publishing requires completed audio processing (the batch endpoint
  reports skipped tracks).
- An unpublished track is only visible to its owner. A WherePublished /
  VisibleTo filter is applied to search, genres, listening history, last
  listened, likes, mixes, album and playlist contents and counts, recently
  listened albums, public user tracks, track details and stream tickets.
- isPublished is part of the track responses, GET /tracks/users/{userId}
  accepts a published filter and status-counts reports published and
  unpublished totals.
- CreateTrackRequest.pictureIntentId is optional: without it the track
  uses the album cover or the preset picture and no picture processing
  runs. CreateTrackResourcesEvent carries nullable picture fields, so
  queues must be drained before deploying.
- POST /tracks/upload-urls accepts an audios list and one optional picture
  shared by all of them, and returns an intent per file. The upload intent
  limits are raised so a batch fits.

Albums
- AlbumType (Album, EP, Single) on create, update and responses.
- CreateAlbumRequest.trackIds creates the album and links the tracks in a
  single save.

backend/Core/Musify.Application/Tracks/SetTrackPublished.cs

@@ -0,0 +1,38 @@
+using ErrorOr;
+using Mediator;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Shared;
+
+namespace Musify.Application.Tracks
+{
+    public record SetTrackPublishedCommand(long UserId, Guid TrackId, bool IsPublished)
+        : ICommand<ErrorOr<Success>>;
+
+    public class SetTrackPublishedCommandHandler(
+        IDatabase database,
+        ILogger<SetTrackPublishedCommandHandler> logger)
+        : ICommandHandler<SetTrackPublishedCommand, ErrorOr<Success>>
+    {
+        public async ValueTask<ErrorOr<Success>> Handle(SetTrackPublishedCommand request, CancellationToken cancellationToken)
+        {
+            var found = await database.Tracks.FindOwnedAsync(request.TrackId, request.UserId, cancellationToken);
+            if (found.IsError)
+                return found.Errors;
+
+            var track = found.Value;
+            if (request.IsPublished && !track.Audio.IsProcessed)
+                return Error.Validation("Track.NotProcessed", "Only tracks whose audio finished processing can be published.");
+
+            if (track.IsPublished == request.IsPublished)
+                return Result.Success;
+
+            track.IsPublished = request.IsPublished;
+            await database.SaveChangesAsync(cancellationToken);
+
+            logger.LogInformation("{Action} track {TrackId}", request.IsPublished ? "Published" : "Unpublished", track.Id);
+
+            return Result.Success;
+        }
+    }
+}