Add Slack-style emoji reactions to comments and issue descriptions with
full-stack support: database tables, REST API endpoints, real-time
WebSocket sync, optimistic UI updates, and inbox notifications.
- New `comment_reaction` and `issue_reaction` tables with migrations
- POST/DELETE endpoints for adding/removing reactions on both comments
and issue descriptions
- Real-time WS events (reaction:added/removed, issue_reaction:added/removed)
- Shared ReactionBar component with quick emoji picker and full emoji-mart
picker (lazy-loaded)
- Optimistic add/remove with rollback on failure
- Inbox notifications for comment author and issue creator when reacted to
- Reactions included in timeline, comment list, and issue detail responses
