feat(reactions): add emoji reactions for comments and issue descriptions

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

server/pkg/db/queries/reaction.sql

@@ -0,0 +1,14 @@
+-- name: AddReaction :one
+INSERT INTO comment_reaction (comment_id, workspace_id, actor_type, actor_id, emoji)
+VALUES ($1, $2, $3, $4, $5)
+ON CONFLICT (comment_id, actor_type, actor_id, emoji) DO UPDATE SET created_at = comment_reaction.created_at
+RETURNING *;
+
+-- name: RemoveReaction :exec
+DELETE FROM comment_reaction
+WHERE comment_id = $1 AND actor_type = $2 AND actor_id = $3 AND emoji = $4;
+
+-- name: ListReactionsByCommentIDs :many
+SELECT * FROM comment_reaction
+WHERE comment_id = ANY($1::uuid[])
+ORDER BY created_at ASC;