Add :collection to chat entity enum

priv/repo/migrations/20260413090350_add_collection_entity_to_chats.exs

@@ -0,0 +1,8 @@
+defmodule MusicLibrary.Repo.Migrations.AddCollectionEntityToChats do
+  use Ecto.Migration
+
+  # The entity column is a plain string in SQLite — no CHECK constraint to alter.
+  # This migration documents the addition of the :collection entity type.
+  def change do
+  end
+end