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>

README.md

@@ -237,7 +237,7 @@ Reddit blocks reader view, so a meme shows the picture from the feed.
 lib/
   main.dart                    App entry, theme
   data/default_sources.dart    Curated feed list
-  models/                      Article, NewsSource, DailyBriefing, AppSettings
+  models/                      Article, Source, DailyBriefing, AppSettings
   services/
     feed_parser.dart           RSS 2.0 / RSS 1.0 / Atom parser, date parsing
     feed_service.dart          Parallel fetching, dedupe, time window