Add first version of online stores for wishlisted records (claude)

See docs/plans/online_stores.md

priv/repo/migrations/20250715082338_create_online_store_templates.exs

@@ -0,0 +1,17 @@
+defmodule MusicLibrary.Repo.Migrations.CreateOnlineStoreTemplates do
+  use Ecto.Migration
+
+  def change do
+    create table(:online_store_templates, primary_key: false) do
+      add :id, :binary_id, primary_key: true
+      add :name, :string, null: false
+      add :description, :text
+      add :url_template, :text, null: false
+      add :enabled, :boolean, default: true, null: false
+
+      timestamps(type: :utc_datetime)
+    end
+
+    create index(:online_store_templates, [:enabled])
+  end
+end