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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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

@@ -15,7 +15,8 @@ namespace Musify.Application.Services
         PictureRoutes Routes,
         string FileType,
         string ContentType,
-        long? ExpectedSizeBytes);
+        long? ExpectedSizeBytes,
+        int? SharedKeyGroup = null);
 
     public sealed record IssuedUpload(
         Guid IntentId,
@@ -43,11 +44,15 @@ namespace Musify.Application.Services
             if (!userExists)
                 return AppErrors.NotFound("User", userId);
 
+            var sharedKeys = new Dictionary<int, string>();
             var planned = requests
                 .Select(request =>
                 {
                     var objectName = $"{Guid.NewGuid()}.{request.FileType.TrimStart('.').ToLowerInvariant()}";
                     var key = request.Routes.BuildTempPath(uploadIntentConfiguration.TempRootPrefix, userId, objectName);
+                    if (request.SharedKeyGroup is { } group)
+                        key = sharedKeys.TryAdd(group, key) ? key : sharedKeys[group];
+
                     var size = request.ExpectedSizeBytes ?? DefaultSize(request.Purpose);
                     return (Request: request, ObjectName: objectName, Key: key, Size: size);
                 })