ML-194: add selective smoke coverage

Add smoke tests for bulk cron workers, CSP browser header, and
on_mount hooks (GetTimezone/StaticAssets). 9 tests across 4 files,
all passing alongside 135 existing related tests.

test/music_library_web/hooks/static_assets_test.exs

@@ -0,0 +1,14 @@
+defmodule MusicLibraryWeb.Hooks.StaticAssetsTest do
+  use MusicLibraryWeb.ConnCase
+
+  import Phoenix.LiveViewTest, only: [live: 2]
+
+  describe "on_mount/4" do
+    test "assigns :static_changed to a boolean", %{conn: conn} do
+      {:ok, view, _html} = live(conn, ~p"/collection")
+      socket = :sys.get_state(view.pid).socket
+
+      assert is_boolean(socket.assigns.static_changed)
+    end
+  end
+end