Permissions & collaboration model #4

Description

Teams self-hosting docsgraph-server need to control who can see and edit which documents, and collaborate without stepping on each other.

Proposed Solution

Implement a permissions model (users, roles/groups, per-document or per-collection access) enforced on all sync and API operations.

Acceptance Criteria

• Users and roles/groups can be defined
• Document/collection-level access control is enforced on read and write
• Sync only delivers changes the requesting user is permitted to see
• Permission checks are covered by tests