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/source.dart

@@ -0,0 +1,73 @@
+enum SourceLanguage {
+  de('Deutsch', '🇩🇪'),
+  en('English', '🇬🇧');
+
+  const SourceLanguage(this.label, this.flag);
+
+  final String label;
+  final String flag;
+}
+
+/// What a Source's articles are for; see GLOSSARY.md. Each kind has its own
+/// section in the app and its own time window.
+enum SourceKind {
+  news('News'),
+  learning('Learning'),
+  fun('Fun');
+
+  const SourceKind(this.label);
+
+  final String label;
+}
+
+/// A Source of any [kind]; see GLOSSARY.md.
+class Source {
+  const Source({
+    required this.id,
+    required this.name,
+    required this.feedUrl,
+    required this.homepage,
+    required this.language,
+    this.kind = SourceKind.news,
+    this.description = '',
+    this.enabledByDefault = false,
+    this.isCustom = false,
+    this.checkPaywallOnPage = false,
+  });
+
+  final String id;
+  final String name;
+  final String feedUrl;
+  final String homepage;
+  final SourceLanguage language;
+  final SourceKind kind;
+  final String description;
+  final bool enabledByDefault;
+  final bool isCustom;
+
+  /// Paid articles are only marked inside the page (schema.org), not in the
+  /// feed, so each new article page is checked once.
+  final bool checkPaywallOnPage;
+
+  Map<String, dynamic> toJson() => {
+        'id': id,
+        'name': name,
+        'feedUrl': feedUrl,
+        'homepage': homepage,
+        'language': language.name,
+        'kind': kind.name,
+        'description': description,
+      };
+
+  factory Source.custom(Map<String, dynamic> json) => Source(
+        id: json['id'] as String,
+        name: json['name'] as String,
+        feedUrl: json['feedUrl'] as String,
+        homepage: json['homepage'] as String? ?? '',
+        language: SourceLanguage.values.byName(json['language'] as String? ?? 'en'),
+        // Custom sources saved before kinds existed are News sources.
+        kind: SourceKind.values.asNameMap()[json['kind']] ?? SourceKind.news,
+        description: json['description'] as String? ?? '',
+        isCustom: true,
+      );
+}