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/services/summarizer.dart

@@ -1,6 +1,6 @@
 import '../models/article.dart';
 import '../models/briefing.dart';
-import '../models/news_source.dart';
+import '../models/source.dart';
 
 class SummarizerException implements Exception {
   SummarizerException(this.message);