Add anti-entropy repair for full-replica cache replication

Summary

Add a background repair mechanism so full-replica regions periodically compare state and heal missed writes, delayed deletes, or partial bootstrap gaps.

Why

Outbox retries help with transient delivery failures, but they do not replace periodic reconciliation. A repair pass is the safety net that makes eventual consistency converge.

Scope

• choose a repair unit (manifest range, time window, project prefix, or hash partition)
• compare local and remote manifests efficiently
• copy missing blobs and reconcile stale metadata
• make repair incremental and rate-limited so it does not impact hot build traffic
• expose repair metrics and add e2e coverage for healing a deliberately diverged node

Notes

This should complement, not replace, normal asynchronous replication.