Sync protocol & service implementation #2

Description

docsgraph clients need to synchronize their local-first document store with the server, including catching up after being offline.

Proposed Solution

Design and implement a sync protocol (push/pull of changes, versioning/change tracking) that the docsgraph client can integrate against.

Acceptance Criteria

• Sync protocol is documented (request/response shapes, auth)
• Server can accept pushed changes and serve pulled changes since a given point
• Protocol supports clients reconnecting after extended offline periods
• Basic conflict/versioning strategy is defined