Collab Phase 6: Idempotent Overwrite and Conflict Payload Enhancements for Deferred Commit Clients

## Summary
Enhance RERUM durability APIs for deferred commit workloads with idempotency and richer conflict metadata.

## Scope
- Support idempotency keys for overwrite and retry safety.
- Ensure conflict responses include current revision and current document metadata.
- Evaluate optional bulk overwrite endpoint for batched page flushes.
- Emit metrics for write rate, conflict rate, and latency.

## Acceptance Criteria
- Retries using same idempotency key do not duplicate writes.
- Conflict response contains fields needed for client reconciliation.
- Metrics are emitted for durability observability.

## Dependency
- Aligns with TPEN services durable flush worker issue.