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/entities/like.py

@@ -0,0 +1,40 @@
+"""Domain entity for PostLike.
+
+This module defines the PostLike entity that tracks which users
+or devices have liked which posts.
+"""
+
+from dataclasses import dataclass
+from typing import Any
+from uuid import UUID
+
+from app.domain.entities.base import BaseEntity
+
+
+@dataclass(kw_only=True)
+class PostLike(BaseEntity):
+    """Post like domain entity.
+
+    Tracks a like on a blog post by a user or device.
+    Each like is uniquely identified by its entity ID.
+
+    Attributes:
+        post_id: UUID of the liked post.
+        liked_by: Identifier of the user or device that liked.
+    """
+
+    post_id: UUID
+    liked_by: str
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert entity to dictionary.
+
+        Returns:
+            Dictionary with all PostLike attributes.
+        """
+        return {
+            "id": str(self.id),
+            "post_id": str(self.post_id),
+            "liked_by": self.liked_by,
+            "created_at": self.created_at.isoformat(),
+        }