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

## 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