Design Slack-like team chat

Design a Slack-like team messaging system. Include: - Core features: workspaces/organizations, channels, 1:1 and group DMs, send/receive messages in real time, message history sync across devices. - Common features (prioritize and justify): threading, reactions, @mentions, file uploads, search, notifications, presence/typing indicators, message edit/delete, retention. - Non-functional requirements: low latency delivery, scalability to large workspaces and large channels, high availability, reasonable ordering guarantees, multi-device consistency, security and access control. Describe: - APIs (write/send, read/history, subscribe/stream, search). - Data model and storage choices. - Real-time delivery architecture (e.g., WebSockets), fanout strategy, and how clients catch up after being offline. - How you handle ordering, deduplication, retries, and multi-region deployment. - Monitoring and operational concerns.