feat: add entity type enum

- Define entity types
- Add category enum
- Add source enum

Author: fulleni

lib/entity_details/models/entity_type.dart

@@ -0,0 +1,8 @@
+/// Defines the type of entity being displayed or interacted with.
+enum EntityType {
+  /// Represents a news category.
+  category,
+
+  /// Represents a news source.
+  source,
+}