feat: add like/unlike toggle on blog posts with per-user tracking

- PostLike domain entity (post_id, liked_by) with BaseEntity integration
- Post entity: add like_count field (default 0) and to_dict serialization
- PostRepository interface: add get_like, add_like, remove_like methods
- TogglePostLikeUseCase: toggle logic (like → unlike, unlike → like)
- PostResponseDTO/PostResponseSchema: add like_count field
- PostLikeORM model with FK to posts and cascade delete
- SQLAlchemyPostRepository: implement like query/add/remove with ORM mapping
- DI provider registration for TogglePostLikeUseCase
- API endpoint POST /api/v1/posts/{id}/like (auth required)
- Unit tests: PostLike entity, Post.like_count, TogglePostLikeUseCase (7 tests)
- API tests: POST /api/v1/posts/{id}/like (4 tests)
- Test model files: FEATURE_LIKES.md, TEST_MODEL.md updated

app/domain/__init__.py

@@ -4,7 +4,7 @@ This module re-exports all domain layer components including
 entities, value objects, repositories, and exceptions.
 """
 
-from app.domain.entities import BaseEntity, Post
+from app.domain.entities import BaseEntity, Post, PostLike
 from app.domain.exceptions import (
     AlreadyExistsException,
     DomainException,
@@ -19,6 +19,7 @@ from app.domain.value_objects import Content, Slug, Title, ValueObject
 __all__ = [
     "BaseEntity",
     "Post",
+    "PostLike",
     "ValueObject",
     "Title",
     "Content",