docs(README): update with new real-time, queue, and caching features

Update project description, feature list, and architecture diagram to reflect newly added scalable background job processing, real-time WebSocket tracking, BullMQ queues, and Redis (Upstash) caching/pub/sub/queue backend. Also update related operational details: 25-second SSE heartbeat for in-app notifications, 3-attempt exponential backoff retry for email delivery, and restructure the platform features table to separate real-time capabilities for improved clarity.

Author: Hamid