feat: add support for alternative ingredients; implement JSON storage and parsing logic

flutter/lib/features/recipes/domain/parsed_recipe.dart

@@ -23,6 +23,7 @@ class ParsedIngredient {
   final String unit;
   final String? note;
   final List<IngredientSuggestion> suggestions;
+  final List<String> alternatives;
 
   const ParsedIngredient({
     required this.rawName,
@@ -30,6 +31,7 @@ class ParsedIngredient {
     required this.unit,
     this.note,
     required this.suggestions,
+    this.alternatives = const [],
   });
 
   factory ParsedIngredient.fromJson(Map<String, dynamic> json) {
@@ -42,6 +44,9 @@ class ParsedIngredient {
       suggestions: rawSuggestions
           .map((s) => IngredientSuggestion.fromJson(s as Map<String, dynamic>))
           .toList(),
+      alternatives: (json['alternatives'] as List<dynamic>? ?? [])
+          .map((a) => a as String)
+          .toList(),
     );
   }
 }