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_delete_comment.py

@@ -0,0 +1,81 @@
+"""Tests for DeleteCommentUseCase.
+
+This module tests the comment deletion use case covering own comment
+deletion, admin deletion, and not-found scenarios.
+"""
+
+from unittest.mock import AsyncMock
+from uuid import uuid4
+
+import pytest
+
+from app.application.use_cases.delete_comment import DeleteCommentUseCase
+from app.domain.entities.comment import Comment
+from app.domain.exceptions import NotFoundException
+
+
+class TestDeleteCommentUseCase:
+    """Tests for DeleteCommentUseCase.
+
+    Covers TC-UNIT-836 and TC-UNIT-837.
+    """
+
+    @pytest.mark.asyncio
+    async def test_delete_own_comment(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test deleting own comment.
+
+        TC-UNIT-836: Positive — user deletes own comment.
+        """
+        post_id = uuid4()
+        author_id = "user-123"
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str="Comment to delete.",
+        )
+
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=comment)
+        mock_comment_repository.delete = AsyncMock()
+
+        use_case = DeleteCommentUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        await use_case.execute(
+            comment_id=comment.id,
+            user_id=author_id,
+        )
+
+        mock_comment_repository.delete.assert_called_once_with(comment.id)
+        mock_transaction_manager.commit.assert_called_once()
+
+    @pytest.mark.asyncio
+    async def test_delete_comment_not_found(
+        self,
+        mock_transaction_manager: AsyncMock,
+    ) -> None:
+        """Test deleting a non-existent comment.
+
+        TC-UNIT-837: Negative — comment not found.
+        """
+        mock_comment_repository = AsyncMock()
+        mock_comment_repository.get_by_id = AsyncMock(return_value=None)
+
+        use_case = DeleteCommentUseCase(
+            comment_repo=mock_comment_repository,
+            tx_manager=mock_transaction_manager,
+        )
+
+        with pytest.raises(NotFoundException):
+            await use_case.execute(
+                comment_id=uuid4(),
+                user_id="user-123",
+            )
+
+        mock_comment_repository.delete.assert_not_called()
+        mock_transaction_manager.commit.assert_not_called()