Offline-first collaborative sync

Design the real-time sync backend for a collaborative offline-first app (think a shared notes/whiteboard tool) where 10M users edit documents that must converge across devices, work offline, and merge without a central lock. Edits stream over WebSocket when online and queue locally when offline. You need <500 ms propagation between online collaborators on a doc and automatic conflict resolution. How do edits flow, and how do you guarantee convergence?