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.

app/presentation/schemas/comment.py

@@ -0,0 +1,58 @@
+"""Pydantic schemas for comments.
+
+This module defines Pydantic models for comment request/response
+validation in the API layer.
+"""
+
+from datetime import datetime
+from uuid import UUID
+
+from pydantic import BaseModel, Field
+
+
+class CommentCreateSchema(BaseModel):
+    """Schema for creating a comment.
+
+    Attributes:
+        content: Comment text content (Markdown supported).
+        parent_id: Optional parent comment ID for replies.
+    """
+
+    content: str = Field(..., min_length=1, max_length=5000, description="Comment content")
+    parent_id: UUID | None = Field(default=None, description="Parent comment ID for replies")
+
+
+class CommentResponseSchema(BaseModel):
+    """Schema for comment response.
+
+    Attributes:
+        id: Unique comment identifier.
+        post_id: UUID of the parent post.
+        author_id: Comment author identifier.
+        content: Comment content text.
+        parent_id: Optional parent comment ID.
+        like_count: Number of likes on this comment.
+        created_at: Creation timestamp.
+        updated_at: Last update timestamp.
+    """
+
+    id: UUID
+    post_id: UUID
+    author_id: str
+    content: str
+    parent_id: UUID | None = None
+    like_count: int = 0
+    created_at: datetime | None = None
+    updated_at: datetime | None = None
+
+
+class CommentLikeResponseSchema(BaseModel):
+    """Schema for comment like response.
+
+    Attributes:
+        id: Comment identifier.
+        like_count: Updated like count.
+    """
+
+    id: UUID
+    like_count: int