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/infrastructure/database/models.py

@@ -4,10 +4,12 @@ This module defines the database ORM models that map to database tables.
 Models are used by repositories for data persistence.
 """
 
+from __future__ import annotations
+
 from datetime import UTC, datetime
 from uuid import uuid4
 
-from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, String, Text
+from sqlalchemy import JSON, Boolean, DateTime, ForeignKey, Integer, String, Text
 from sqlalchemy.orm import Mapped, declarative_base, mapped_column, relationship
 
 Base = declarative_base()
@@ -43,7 +45,10 @@ class PostORM(Base):  # type: ignore[valid-type,misc]
     author_id: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
     published: Mapped[bool] = mapped_column(Boolean, default=False, nullable=False, index=True)
     like_count: Mapped[int] = mapped_column(default=0, nullable=False)
-    likes: Mapped[list["PostLikeORM"]] = relationship(
+    likes: Mapped[list[PostLikeORM]] = relationship(
+        back_populates="post", cascade="all, delete-orphan"
+    )
+    comments: Mapped[list[CommentORM]] = relationship(
         back_populates="post", cascade="all, delete-orphan"
     )
     tags: Mapped[list[str]] = mapped_column(JSON, default=list)
@@ -87,3 +92,87 @@ class PostLikeORM(Base):  # type: ignore[valid-type,misc]
     )
 
     post: Mapped[PostORM] = relationship(back_populates="likes")
+
+
+class CommentORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for Comment.
+
+    Database table representation of comments on blog posts.
+    Maps to the 'comments' table with support for nested replies.
+
+    Attributes:
+        id: Primary key as UUID string.
+        post_id: Foreign key to the parent post.
+        author_id: Comment author reference.
+        content: Comment text content (Markdown supported).
+        parent_id: Optional foreign key to parent comment (replies).
+        like_count: Number of likes on this comment.
+        created_at: Creation timestamp.
+        updated_at: Last update timestamp.
+    """
+
+    __tablename__ = "comments"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid4()))
+    post_id: Mapped[str] = mapped_column(
+        String(36), ForeignKey("posts.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    author_id: Mapped[str] = mapped_column(String(100), nullable=False, index=True)
+    content: Mapped[str] = mapped_column(Text, nullable=False)
+    parent_id: Mapped[str | None] = mapped_column(
+        String(36),
+        ForeignKey("comments.id", ondelete="SET NULL"),
+        nullable=True,
+        index=True,
+    )
+    like_count: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        onupdate=lambda: datetime.now(UTC),
+        nullable=False,
+    )
+
+    post: Mapped[PostORM] = relationship(back_populates="comments")
+    replies: Mapped[list[CommentORM]] = relationship(
+        back_populates="parent",
+    )
+    parent: Mapped[CommentORM | None] = relationship(
+        back_populates="replies",
+        remote_side="CommentORM.id",
+    )
+
+
+class CommentLikeORM(Base):  # type: ignore[valid-type,misc]
+    """SQLAlchemy model for CommentLike.
+
+    Database table representation of comment likes.
+    Maps to the 'comment_likes' table tracking which users liked which comments.
+
+    Attributes:
+        id: Primary key as UUID string.
+        comment_id: Foreign key to the liked comment.
+        liked_by: User identifier.
+        created_at: Creation timestamp.
+    """
+
+    __tablename__ = "comment_likes"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid4()))
+    comment_id: Mapped[str] = mapped_column(
+        String(36),
+        ForeignKey("comments.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    liked_by: Mapped[str] = mapped_column(String(200), nullable=False, index=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(UTC),
+        nullable=False,
+    )