feat: implement Infra layer for user favorites repository / お気に入り機能のInfra層実装

[zigzagdev]

Motivation

Following the domain-layer migration for the user_favorite join table (#560), this PR implements the Infra layer for the favorites feature: a FavoriteRepositoryInterface and its Eloquent-backed FavoriteRepository implementation, allowing a user to add a world heritage site to their favorites.

What I have done

• Added a User::favorites() BelongsToMany relation to WorldHeritage via the user_favorite pivot table
• Added FavoriteRepositoryInterface with addFavorite(int $userId, int $worldHeritageSiteId): void
• Added FavoriteRepository, implementing the interface via $user->favorites()->attach(), throwing an exception when the user is not found
• Bound FavoriteRepositoryInterface to FavoriteRepository in RepositoryProvider
• Added an integration test for FavoriteRepository against the real test database

Test Results

• test_addFavorite_attaches_world_heritage_site_to_user
• test_addFavorite_throws_exception_when_user_not_found

[zigzagdev]

OK