Collab Phase 5: Durable Flush Worker from Drafts to RERUM with Conflict Acks

[cubap]

Summary

Create server-side durable flush pipeline that coalesces draft operations and commits to RERUM with optimistic locking.

Scope

• Batch and coalesce queued operations by line.
• Persist through existing Line/Page update flow.
• Use optimistic locking for overwrite conflict detection.
• Emit durable events: queued, started, persisted, conflict, failed.
• Return durable revision metadata in ack payload.

Acceptance Criteria

• Multiple rapid edits are coalesced into fewer durable writes.
• 409 conflicts are emitted as durable conflict events with context.
• Persisted acknowledgements include durable revision references.
• Backpressure limits prevent unbounded growth.

Dependencies

• Depends on draft intake issue.
• Contract: docs/collaboration/websocket-collaboration-contract.md