Fix stale PubSub subscriptions on record navigation

Subscription management was only done in mount/3, meaning
navigating to a different record via handle_params/3 left the
old subscription active. A background update for the old record
would then overwrite the currently displayed record.

Fix: manage subscriptions in handle_params/3 (unsubscribe old,
subscribe new) via shared RecordActions.manage_subscription/2,
and guard handle_info({:update,...}) against mismatched IDs.

lib/music_library/records.ex

@@ -110,6 +110,14 @@ defmodule MusicLibrary.Records do
     Phoenix.PubSub.subscribe(MusicLibrary.PubSub, "records:#{record_id}")
   end
 
+  @doc """
+  Unsubscribes the calling process from updates for a given record.
+  """
+  @spec unsubscribe(String.t()) :: :ok | {:error, term()}
+  def unsubscribe(record_id) do
+    Phoenix.PubSub.unsubscribe(MusicLibrary.PubSub, "records:#{record_id}")
+  end
+
   @spec notify_update(Record.t()) :: :ok | {:error, term()}
   def notify_update(record) do
     Phoenix.PubSub.broadcast(