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_like_entity.py

@@ -0,0 +1,50 @@
+"""Tests for PostLike domain entity.
+
+This module tests the PostLike entity creation, attributes,
+and BaseEntity integration.
+"""
+
+from uuid import UUID
+
+from app.domain.entities.like import PostLike
+
+
+class TestPostLikeEntity:
+    """Tests for the PostLike domain entity.
+
+    Covers TC-UNIT-826: PostLike entity valid creation.
+    """
+
+    def test_post_like_creation(self) -> None:
+        """Test creating a PostLike with valid attributes.
+
+        TC-UNIT-826: Positive — create PostLike instance.
+
+        Expected:
+            - post_id matches input
+            - liked_by matches input
+            - id is a valid UUID
+            - created_at is set
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "user-123"
+
+        like = PostLike(post_id=post_id, liked_by=liked_by)
+
+        assert like.post_id == post_id
+        assert like.liked_by == liked_by
+        assert isinstance(like.id, UUID)
+        assert like.created_at is not None
+
+    def test_post_like_to_dict(self) -> None:
+        """Test PostLike to_dict serialization."""
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        liked_by = "device-abc-123"
+
+        like = PostLike(post_id=post_id, liked_by=liked_by)
+        data = like.to_dict()
+
+        assert data["post_id"] == str(post_id)
+        assert data["liked_by"] == liked_by
+        assert "id" in data
+        assert "created_at" in data