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

tests/unit/domain/test_entities.py

@@ -128,6 +128,19 @@ class TestPost:
         assert "created_at" in data
         assert "updated_at" in data
 
+    def test_like_count_defaults_to_zero(self) -> None:
+        """Test that a new post has like_count defaulting to 0.
+
+        TC-UNIT-827: Positive — like_count defaults to zero on creation.
+        """
+        post = Post.create(
+            title_str="Test Post",
+            content_str="This is test content that is long enough",
+            author_id="user-123",
+        )
+
+        assert post.like_count == 0
+
     def test_base_entity_eq_and_hash(self) -> None:
         """Test BaseEntity equality and hash directly."""
         from app.domain.entities.base import BaseEntity