ML-168: broadcast index_changed event after background import

Import workers now broadcast :records_index_changed on
"records:index_changed" after successful import via
Records.broadcast_index_changed/0.

CollectionLive.Index and WishlistLive.Index subscribe to the
topic in mount/3 and reload their record streams on receipt,
with a live_action guard to skip reloads when the grid is
hidden behind a modal (:import, :barcode_scan).

IndexActions.handle_index_changed/1 refreshes total_entries
before reloading to keep the pagination bar accurate.

lib/music_library/records.ex

@@ -126,4 +126,21 @@ defmodule MusicLibrary.Records do
       {:update, record}
     )
   end
+
+  @doc """
+  Broadcasts that the records index has changed (new record imported, deleted, etc.).
+  Index LiveViews subscribe to this topic to auto-refresh.
+  """
+  @spec broadcast_index_changed() :: :ok
+  def broadcast_index_changed do
+    Phoenix.PubSub.broadcast(MusicLibrary.PubSub, "records:index_changed", :records_index_changed)
+  end
+
+  @doc """
+  Subscribes the calling process to records index change notifications.
+  """
+  @spec subscribe_to_index() :: :ok | {:error, term()}
+  def subscribe_to_index do
+    Phoenix.PubSub.subscribe(MusicLibrary.PubSub, "records:index_changed")
+  end
 end