Make album cover pictures mandatory, unify cover handling with playlists

Albums previously had no cover concept at all: no picture upload
flow, no GET /albums/{id}/cover, and AlbumApplicationResponse's
image-key fields were plumbed but never populated. Description,
Pictures and TrackCount already lived on the shared Album base entity
for both local and external albums, so unifying the cover story only
required building the missing local-album picture pipeline.

Backend, cloning PlayList's existing async picture-processing
architecture exactly (MassTransit routing slip + saga), but making
the picture mandatory instead of optional (matching Track's pattern):

- AlbumPictures moves from all-nullable to required fields.
- New AlbumConfiguration (routes/sizes), deliberately without
  PlayList's preset-picture fallback since a picture is now required
  up front.
- CreateAlbum now takes a mandatory PictureIntentId, validates and
  consumes the upload intent, and kicks off the same
  copy-to-final -> resize -> upload -> update-entity routing slip
  PlayLists use, via a new AlbumProcessingState saga that marks the
  album Failed on a picture-processing failure.
- UpdateAlbum takes an optional NewPictureIntentId to replace the
  cover later.
- New GET /albums/{id}/cover and POST /albums/upload-picture
  endpoints, mirroring the playlist ones.
- New EF migration for the AlbumProcessingState saga table.

Frontend:

- AlbumForm gains a cover dropzone, required on create and optional
  on edit (via ImageDropzone's new `required` prop), uploaded through
  the same presigned-upload helper playlists use.
- New /api/albums/{id}/cover proxy route.
- PlaylistArt gains a generic coverUrl prop so AlbumHeader and the
  album list/detail cards render the real uploaded cover instead of
  only the track mosaic fallback.

Delete-time picture cleanup for albums is intentionally left as a
follow-up, matching the existing gap in playlist deletion.

backend/Core/Musify.Infrastructure/MassTransit/Arguments/MarkAlbumAsFailedArguments.cs

@@ -0,0 +1,4 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record MarkAlbumAsFailedArguments(Guid AlbumId);
+}

backend/Core/Musify.Infrastructure/MassTransit/Arguments/PublishAlbumPictureProcessingEventArguments.cs

@@ -0,0 +1,12 @@
+using Musify.Application.Shared;
+
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record PublishAlbumPictureProcessingEventArguments(
+        Guid AlbumId,
+        string Bucket,
+        string FinalPictureKey,
+        ImageSize Small,
+        ImageSize Medium,
+        ImageSize Large);
+}

backend/Core/Musify.Infrastructure/MassTransit/Arguments/UpdateAlbumPictureArguments.cs

@@ -0,0 +1,9 @@
+namespace Musify.Infrastructure.MassTransit.Arguments
+{
+    public record UpdateAlbumPictureArguments(
+        Guid AlbumId,
+        string OriginalPictureKey,
+        string SmallPictureVariable,
+        string MediumPictureVariable,
+        string LargePictureVariable);
+}