feat: add comments feature with nested replies and recursive rendering

Implement full comments system: domain entities (Comment, CommentLike),
value objects (CommentContent), use cases (CRUD, like toggle), SQLAlchemy
repository, API v1 endpoints, web UI with comment form and nested replies,
i18n translations (EN/RU/FR/DE), and E2E tests.

Fix nested reply (reply-to-reply) not displaying — the flat reply_comments
dict was only queried for top-level comment IDs, so deeply nested replies
were saved to DB (incrementing comment count) but never rendered. Switch
to a recursive Jinja2 macro that renders any nesting depth.

tests/unit/application/test_toggle_comment_like.py

@@ -0,0 +1,119 @@
+"""Tests for ToggleCommentLikeUseCase.
+
+This module tests the comment like/unlike toggle use case covering
+first-time like, unlike, and comment-not-found scenarios.
+"""
+
+from unittest.mock import AsyncMock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.toggle_comment_like import ToggleCommentLikeUseCase
+from app.domain.entities.comment import Comment
+from app.domain.entities.comment_like import CommentLike
+from app.domain.exceptions import NotFoundException
+
+
+class TestToggleCommentLikeUseCase:
+    """Tests for ToggleCommentLikeUseCase.
+
+    Covers TC-UNIT-838 through TC-UNIT-840.
+    """
+
+    @pytest.mark.asyncio
+    async def test_like_comment_first_time(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test liking a comment for the first time.
+
+        TC-UNIT-838: Positive — like first time.
+        """
+        post_id = uuid4()
+        comment = Comment.create(
+            post_id=post_id,
+            author_id="user-123",
+            content_str="Nice post!",
+        )
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.get_like = AsyncMock(return_value=None)
+        mock_comment_repository.add_like = AsyncMock()
+        mock_comment_repository.remove_like = AsyncMock()
+        mock_comment_repository.update = AsyncMock()
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(comment.id, "user-456")
+
+        assert result.like_count == 1
+        mock_comment_repository.add_like.assert_called_once()
+        mock_comment_repository.remove_like.assert_not_called()
+        mock_comment_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_unlike_comment_already_liked(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test unliking a comment that is already liked.
+
+        TC-UNIT-839: Positive — unlike (already liked).
+        """
+        post_id = uuid4()
+        comment = Comment.create(
+            post_id=post_id,
+            author_id="user-123",
+            content_str="Nice post!",
+        )
+        existing_like = CommentLike(comment_id=comment.id, liked_by="user-456")
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.get_like = AsyncMock(return_value=existing_like)
+        mock_comment_repository.add_like = AsyncMock()
+        mock_comment_repository.remove_like = AsyncMock()
+        mock_comment_repository.update = AsyncMock()
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        result = await use_case.execute(comment.id, "user-456")
+
+        assert result.like_count == 0
+        mock_comment_repository.remove_like.assert_called_once()
+        mock_comment_repository.add_like.assert_not_called()
+        mock_comment_repository.update.assert_called_once()
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_like_comment_not_found(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test liking a non-existent comment.
+
+        TC-UNIT-840: Negative — comment not found.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = ToggleCommentLikeUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(uuid4(), "user-456")
+
+        mock_comment_repository.add_like.assert_not_called()
+        mock_comment_repository.remove_like.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()