Use the glossary's words in test names and comments (review item 2)

"Feed" no longer stands for a Source and "newsletter" no longer for a
Learning source; paid_newsletter_posts_test.dart becomes
paid_substack_posts_test.dart.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

lib/services/paywall_service.dart

@@ -9,7 +9,7 @@ import 'storage.dart';
 /// Recognizes paid and sponsored articles so the feed can hide them.
 ///
 /// Most paid articles are marked in the title (heise+, Golem's "(g+)").
-/// Substack newsletters cut paid posts short in the feed and end them with a
+/// Substack cuts paid posts short in the feed and ends them with a
 /// "Read more" link to the post itself, so those are recognized from the feed.
 /// SPIEGEL and t3n only mark them in the page's schema.org data
 /// (`"isAccessibleForFree": false`), so for those sources each new article is