Add ConnectionManager for real-time WebSocket communication:
- Message types for progress updates, annotations, conflicts
- Broadcast message structure with source device tracking
- Device connection tracking with user and device metadata
- Automatic broadcast loop with concurrent message delivery
- Connection management (add, remove, get by ID/user)
- Stale connection cleanup (2-minute timeout)
- Connection statistics by device type
- Background cleanup task runs every minute
This implements the core WebSocket infrastructure needed for
Week 9 of the Universal Sync Implementation Guide.
john-okeefe
9bfe14bb38