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/domain/test_comment_entity.py

@@ -0,0 +1,98 @@
+"""Tests for Comment domain entity.
+
+This module tests the Comment entity creation, parent_id support,
+and BaseEntity integration.
+"""
+
+from uuid import UUID, uuid4
+
+from app.domain.entities.comment import Comment
+from app.domain.value_objects.comment_content import CommentContent
+
+
+class TestCommentEntity:
+    """Tests for the Comment domain entity.
+
+    Covers TC-UNIT-829 and TC-UNIT-830.
+    """
+
+    def test_comment_creation(self) -> None:
+        """Test creating a top-level Comment with valid attributes.
+
+        TC-UNIT-829: Positive — create Comment instance.
+
+        Expected:
+            - post_id matches input
+            - author_id matches input
+            - content is CommentContent with correct value
+            - id is a valid UUID
+            - parent_id is None
+            - like_count is 0
+            - created_at is set
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        author_id = "user-123"
+        content_text = "This is a comment with **Markdown** support."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+        )
+
+        assert comment.post_id == post_id
+        assert comment.author_id == author_id
+        assert isinstance(comment.content, CommentContent)
+        assert comment.content.value == content_text
+        assert isinstance(comment.id, UUID)
+        assert comment.parent_id is None
+        assert comment.like_count == 0
+        assert comment.created_at is not None
+
+    def test_comment_with_parent(self) -> None:
+        """Test creating a reply Comment with parent_id.
+
+        TC-UNIT-830: Positive — create Comment with parent_id.
+
+        Expected:
+            - parent_id matches the provided parent comment ID.
+            - All other attributes set correctly.
+        """
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        parent_id = uuid4()
+        author_id = "user-456"
+        content_text = "This is a reply to another comment."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+            parent_id=parent_id,
+        )
+
+        assert comment.parent_id == parent_id
+        assert comment.post_id == post_id
+        assert comment.author_id == author_id
+        assert comment.content.value == content_text
+        assert comment.like_count == 0
+
+    def test_comment_to_dict(self) -> None:
+        """Test Comment to_dict serialization."""
+        post_id = UUID("00000000-0000-0000-0000-000000000001")
+        author_id = "user-123"
+        content_text = "Comment with serialization test."
+
+        comment = Comment.create(
+            post_id=post_id,
+            author_id=author_id,
+            content_str=content_text,
+        )
+        data = comment.to_dict()
+
+        assert data["post_id"] == str(post_id)
+        assert data["author_id"] == author_id
+        assert data["content"] == content_text
+        assert "id" in data
+        assert "created_at" in data
+        assert data["parent_id"] is None
+        assert data["like_count"] == 0