feat(api): expose the album endpoints

Eight routes under /albums covering the listing, the detail, the tracklist and
the owner-only writes, wired to the same validation filter and ErrorOr mapping
the playlists use.

backend/Musify.Api/DataTransferObjects/Albums/CreateAlbumRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.Albums;
+
+public record CreateAlbumRequest(
+    string Title,
+    string? Description,
+    int? ReleaseYear);

backend/Musify.Api/DataTransferObjects/Albums/UpdateAlbumRequest.cs

@@ -0,0 +1,6 @@
+namespace Musify.Api.DataTransferObjects.Albums;
+
+public record UpdateAlbumRequest(
+    string NewTitle,
+    string? NewDescription,
+    int? NewReleaseYear);