feat(feed): add source type/country filtering

- Added country ISO code filtering
- Added source type filtering
- Updated HeadlineFilter model

Author: fulleni

lib/headlines-feed/models/headline_filter.dart

@@ -6,7 +6,12 @@ import 'package:ht_shared/ht_shared.dart';
 /// {@endtemplate}
 class HeadlineFilter extends Equatable {
   /// {@macro headline_filter}
-  const HeadlineFilter({this.categories, this.sources});
+  const HeadlineFilter({
+    this.categories,
+    this.sources,
+    this.selectedSourceCountryIsoCodes,
+    this.selectedSourceSourceTypes,
+  });
 
   /// The list of selected category filters.
   /// Headlines matching *any* of these categories will be included (OR logic).
@@ -16,15 +21,35 @@ class HeadlineFilter extends Equatable {
   /// Headlines matching *any* of these sources will be included (OR logic).
   final List<Source>? sources;
 
+  /// The set of selected country ISO codes for source filtering.
+  final Set<String>? selectedSourceCountryIsoCodes;
+
+  /// The set of selected source types for source filtering.
+  final Set<SourceType>? selectedSourceSourceTypes;
+
   @override
-  List<Object?> get props => [categories, sources];
+  List<Object?> get props => [
+        categories,
+        sources,
+        selectedSourceCountryIsoCodes,
+        selectedSourceSourceTypes,
+      ];
 
   /// Creates a copy of this [HeadlineFilter] with the given fields
   /// replaced with the new values.
-  HeadlineFilter copyWith({List<Category>? categories, List<Source>? sources}) {
+  HeadlineFilter copyWith({
+    List<Category>? categories,
+    List<Source>? sources,
+    Set<String>? selectedSourceCountryIsoCodes,
+    Set<SourceType>? selectedSourceSourceTypes,
+  }) {
     return HeadlineFilter(
       categories: categories ?? this.categories,
       sources: sources ?? this.sources,
+      selectedSourceCountryIsoCodes: selectedSourceCountryIsoCodes ??
+          this.selectedSourceCountryIsoCodes,
+      selectedSourceSourceTypes:
+          selectedSourceSourceTypes ?? this.selectedSourceSourceTypes,
     );
   }
 }