ML-169.8: reduce Stats mount sync queries to 3

Move 7 non-critical queries out of synchronous mount into assign_async
and start_async tasks with skeleton loading placeholders.

Only scalar badge counters (collection count, wishlist count, scrobble
count) remain synchronous. All async sections use <.async_result> with
loading and failed slots. Scrobble activity preserves LiveView streams
via handle_async. On-this-day date changes use AsyncResult.ok/2.

lib/music_library/collection.ex

@@ -27,6 +27,13 @@ defmodule MusicLibrary.Collection do
     Records.search_records_count(base_search(), query)
   end
 
+  @doc "Direct scalar count for collection badge — faster than summing grouped format counts."
+  @spec count() :: non_neg_integer()
+  def count do
+    from(r in Record, where: not is_nil(r.purchased_at))
+    |> Repo.aggregate(:count)
+  end
+
   @spec count_records_by_format() :: [{String.t(), non_neg_integer()}]
   def count_records_by_format do
     q =