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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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.Domain/Entities/Album.cs

@@ -22,6 +22,8 @@ namespace Musify.Domain.Entities
 
         public int? ReleaseYear { get; set; }
 
+        public AlbumType Type { get; set; }
+
         public EntityPictures? Pictures { get; set; }
 
         public required long OwnerUserId { get; set; }

backend/Core/Musify.Domain/Entities/Track.cs

@@ -28,6 +28,8 @@ namespace Musify.Domain.Entities
 
         public bool IsExplicit { get; set; }
 
+        public bool IsPublished { get; set; }
+
         public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
 
         public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;

backend/Core/Musify.Domain/ValueObjects/AlbumType.cs

@@ -0,0 +1,12 @@
+using System.Diagnostics.CodeAnalysis;
+
+namespace Musify.Domain.ValueObjects
+{
+    public enum AlbumType
+    {
+        Album = 0,
+        EP = 1,
+        [SuppressMessage("Naming", "CA1720:Identifier contains type name", Justification = "Part of the public API contract.")]
+        Single = 2
+    }
+}