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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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/Responses/TrackUploadUrlsResponse.cs

@@ -1,16 +1,25 @@
 namespace Musify.Application.Tracks.Responses
 {
+    public record TrackAudioUpload(
+        Guid IntentId,
+        Guid? PictureIntentId,
+        string Key,
+        string Name,
+        string ContentType,
+        string UploadUrl);
+
     public record TrackUploadUrlsResponse(
-        Guid PictureIntentId,
+        Guid? PictureIntentId,
         Guid AudioIntentId,
         string Bucket,
-        string PictureKey,
-        string PictureName,
-        string PictureContentType,
-        string PictureUploadUrl,
+        string? PictureKey,
+        string? PictureName,
+        string? PictureContentType,
+        string? PictureUploadUrl,
         string AudioKey,
         string AudioName,
         string AudioContentType,
         string AudioUploadUrl,
-        int ExpiresInSeconds);
+        int ExpiresInSeconds,
+        IReadOnlyList<TrackAudioUpload> Audios);
 }