feat: refactor API paths for authentication, inventory, and recipes; add contract tests for repositories

flutter/lib/core/api/api_paths.dart

@@ -0,0 +1,23 @@
+class AuthApiPaths {
+  static const login = '/auth/login';
+}
+
+class ProductApiPaths {
+  static const list = '/products';
+}
+
+class RecipeApiPaths {
+  static const list = '/recipes';
+  static String detail(int id) => '/recipes/$id';
+  static String update(int id) => '/recipes/$id';
+  static String remove(int id) => '/recipes/$id';
+  static const parseMarkdown = '/recipes/parse-markdown';
+}
+
+class InventoryApiPaths {
+  static const list = '/inventory';
+  static String update(int id) => '/inventory/$id';
+  static String remove(int id) => '/inventory/$id';
+  static String consume(int id) => '/inventory/$id/consume';
+  static String consumptionHistory(int id) => '/inventory/$id/consumption-history';
+}