I want to add rich discussion to our documents: users should be able to start a comment, reply to create threaded conversations, and react with emojis. Please build a new backend gRPC service (NestJS + MySQL) with methods to create a top-level comment, reply to an existing comment, and add/remove an emoji reaction. The data model should capture comment hierarchy efficiently (e.g., via a parent_comment_id approach), and reads should be fast: implement a single call that returns an entire thread with its nested replies, along with aggregated reaction counts per emoji for each comment.