Improve multi-tenant safety with user-scoped memory deletions

All memory delete operations now honor user_id across every supported database backend. This ensures strict tenant isolation, reduces the risk of accidental cross-user deletions, and strengthens governance for production deployments.

Details

• Consistent enforcement of user scoping across backends
• If you relied on cross-user deletes, review and update your workflows

Who this is for: SaaS and regulated environments with multi-tenant data isolation requirements.