docs(api): add WebSocket real-time sync API documentation
- sync_api.md - Comprehensive WebSocket API guide
Covers:
- Connection and authentication (JWT + device)
- Client→Server messages (subscribe, ping)
- Server→Client messages (progress, conflicts, scan status, etc.)
- Message formats with examples
- Connection lifecycle
- Topics: progress, conflicts, scanner, devices, queue
- JavaScript usage example with reconnection
- Best practices for heartbeat and error handling
Documents bidirectional real-time sync API
john-okeefe
d43bd9526b