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/cmd/server/notification_listeners.go

@@ -473,6 +473,76 @@ func registerNotificationListeners(bus *events.Bus, queries *db.Queries) {
 		}
 	})
 
+	// issue_reaction:added — notify the issue creator
+	bus.Subscribe(protocol.EventIssueReactionAdded, func(e events.Event) {
+		payload, ok := e.Payload.(map[string]any)
+		if !ok {
+			return
+		}
+
+		reaction, ok := payload["reaction"].(handler.IssueReactionResponse)
+		if !ok {
+			return
+		}
+
+		creatorType, _ := payload["creator_type"].(string)
+		creatorID, _ := payload["creator_id"].(string)
+		issueID, _ := payload["issue_id"].(string)
+		issueTitle, _ := payload["issue_title"].(string)
+		issueStatus, _ := payload["issue_status"].(string)
+
+		if creatorType == "" || creatorID == "" {
+			return
+		}
+
+		details, _ := json.Marshal(map[string]string{
+			"emoji": reaction.Emoji,
+		})
+
+		notifyDirect(ctx, queries, bus,
+			creatorType, creatorID,
+			e.WorkspaceID, e, issueID, issueStatus,
+			"reaction_added", "info",
+			issueTitle, "",
+			details,
+		)
+	})
+
+	// reaction:added — notify the comment author
+	bus.Subscribe(protocol.EventReactionAdded, func(e events.Event) {
+		payload, ok := e.Payload.(map[string]any)
+		if !ok {
+			return
+		}
+
+		reaction, ok := payload["reaction"].(handler.ReactionResponse)
+		if !ok {
+			return
+		}
+
+		commentAuthorType, _ := payload["comment_author_type"].(string)
+		commentAuthorID, _ := payload["comment_author_id"].(string)
+		issueID, _ := payload["issue_id"].(string)
+		issueTitle, _ := payload["issue_title"].(string)
+		issueStatus, _ := payload["issue_status"].(string)
+
+		if commentAuthorType == "" || commentAuthorID == "" {
+			return
+		}
+
+		details, _ := json.Marshal(map[string]string{
+			"emoji": reaction.Emoji,
+		})
+
+		notifyDirect(ctx, queries, bus,
+			commentAuthorType, commentAuthorID,
+			e.WorkspaceID, e, issueID, issueStatus,
+			"reaction_added", "info",
+			issueTitle, "",
+			details,
+		)
+	})
+
 	// task:completed — notify all subscribers except the agent
 	bus.Subscribe(protocol.EventTaskCompleted, func(e events.Event) {
 		payload, ok := e.Payload.(map[string]any)

server/cmd/server/router.go

@@ -164,6 +164,8 @@ func NewRouter(pool *pgxpool.Pool, hub *realtime.Hub, bus *events.Bus) chi.Route
 					r.Get("/subscribers", h.ListIssueSubscribers)
 					r.Post("/subscribe", h.SubscribeToIssue)
 					r.Post("/unsubscribe", h.UnsubscribeFromIssue)
+					r.Post("/reactions", h.AddIssueReaction)
+					r.Delete("/reactions", h.RemoveIssueReaction)
 				})
 			})
 
@@ -171,6 +173,8 @@ func NewRouter(pool *pgxpool.Pool, hub *realtime.Hub, bus *events.Bus) chi.Route
 			r.Route("/api/comments/{commentId}", func(r chi.Router) {
 				r.Put("/", h.UpdateComment)
 				r.Delete("/", h.DeleteComment)
+				r.Post("/reactions", h.AddReaction)
+				r.Delete("/reactions", h.RemoveReaction)
 			})
 
 			// Agents