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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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/Tests/Musify.Api.Tests/Endpoints/TrackLibraryEndpointsTests.cs

@@ -19,7 +19,8 @@ namespace Musify.Api.Tests.Endpoints
         private async Task<Guid> SeedTrackAsync(
             long ownerId,
             string title,
-            ProcessingStatus status = ProcessingStatus.Completed)
+            ProcessingStatus status = ProcessingStatus.Completed,
+            bool isPublished = true)
         {
             using var scope = fixture.Services.CreateScope();
             var database = scope.ServiceProvider.GetRequiredService<Database>();
@@ -28,6 +29,7 @@ namespace Musify.Api.Tests.Endpoints
                 Title = title,
                 NormalizedTitle = title.ToUpperInvariant(),
                 OwnerUserId = ownerId,
+                IsPublished = isPublished,
                 Audio = new TrackAudio { TranscodeStatus = status, FolderName = status == ProcessingStatus.Completed ? "folder" : null }
             };
             database.Tracks.Add(track);