Rename NewsSource to Source now that it models every kind (review item 3)

The class and its file (lib/models/source.dart) follow the glossary term.
Persisted JSON keys are unchanged.

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

lib/models/article.dart

@@ -1,4 +1,4 @@
-import 'news_source.dart';
+import 'source.dart';
 
 class Article {
   const Article({
@@ -14,7 +14,7 @@ class Article {
 
   final String title;
   final String link;
-  final NewsSource source;
+  final Source source;
   final DateTime? published;
 
   /// Short plain-text teaser from the feed (description / summary).