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/domain/value_objects/comment_content.py

@@ -0,0 +1,47 @@
+"""Value object for comment content.
+
+This module defines the CommentContent value object that validates
+and encapsulates comment text content.
+"""
+
+from dataclasses import dataclass
+
+from app.domain.value_objects.base import ValueObject
+
+
+@dataclass(frozen=True, slots=True)
+class CommentContent(ValueObject[str]):
+    """Comment content value object.
+
+    Wraps and validates comment content ensuring it meets length
+    requirements and is not empty.
+
+    Attributes:
+        value: The comment content string.
+        MAX_LENGTH: Maximum allowed content length (5000 characters).
+
+    Raises:
+        ValueError: If content is empty or too long.
+
+    Example:
+        >>> content = CommentContent("This is a **bold** comment.")
+        >>> content.value
+        'This is a **bold** comment.'
+    """
+
+    MAX_LENGTH: int = 5000
+
+    def _validate(self) -> None:
+        """Validate comment content.
+
+        Checks that content is a non-empty string within length bounds.
+
+        Raises:
+            ValueError: If content fails validation criteria.
+        """
+        if not isinstance(self.value, str):
+            raise ValueError("Comment content must be a string")
+        if not self.value.strip():
+            raise ValueError("Comment content cannot be empty")
+        if len(self.value) > self.MAX_LENGTH:
+            raise ValueError(f"Comment content must be at most {self.MAX_LENGTH} characters")