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  tracks are created unpublished.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- An unpublished track is only visible to its owner.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  unpublished totals.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

@@ -25,7 +25,13 @@ namespace Musify.Application.Mixes
                 mix => mix.Id,
                 mix => mix.Items.OrderBy(item => item.Position).ToList());
 
-            var coverItems = orderedItemsByMix.Values
+            var visibleTrackIds = await MixItemMapper.VisibleTrackIdsAsync(
+                database, orderedItemsByMix.Values.SelectMany(items => items), cancellationToken);
+            var visibleItemsByMix = orderedItemsByMix.ToDictionary(
+                entry => entry.Key,
+                entry => entry.Value.Where(item => visibleTrackIds.Contains(item.TrackId)).ToList());
+
+            var coverItems = visibleItemsByMix.Values
                 .SelectMany(items => items.Take(MixApplicationResponse.CoverItemCount))
                 .ToList();
             var coverResponses = (await MixItemMapper.ToResponsesAsync(database, coverItems, cancellationToken))
@@ -35,7 +41,7 @@ namespace Musify.Application.Mixes
             var responses = new List<MixApplicationResponse>(mixes.Count);
             foreach (var mix in mixes)
             {
-                var orderedItems = orderedItemsByMix[mix.Id];
+                var orderedItems = visibleItemsByMix[mix.Id];
                 var covers = orderedItems
                     .Take(MixApplicationResponse.CoverItemCount)
                     .Where(item => coverResponses.ContainsKey(item.TrackId))