feat: Clean up orphaned persistent workspace data when a user is deleted in Open WebUI

When a user account is deleted in Open WebUI, their persistent terminal workspace on the orchestrator host is never removed. On the Docker backend this is TERMINALS_DOCKER_DATA_DIR/<user_id> (plus <user_id>/contexts/ for per-chat contexts); on Kubernetes it's the per-user PVC. The orchestrator only learns about users at request time (JWT validation against Open WebUI), so a deletion is invisible to it and the data persists indefinitely.

Why

Data retention / GDPR-style policies require that a deleted user's personal data be deleted too. Today an operator has to manually find and delete the directory, which is easy to miss and leaves no audit trail.

Suggested approach

• Periodic reconciliation in the reaper sweep: list workspace roots under the data dir (or per-user PVCs on K8s) and compare against the current Open WebUI user list (GET /api/v1/auths/ in JWT mode; for API-key mode, document that operators must supply the list or accept manual cleanup).
• Quarantine before delete: move orphaned dirs to a quarantine/ location with a grace period (e.g. new setting TERMINALS_ORPHAN_RETENTION_DAYS, default 30) so accidental deletions and account re-creation are recoverable.
• Log every quarantine/deletion (user id, path, size, timestamp) for audit.
• Optional admin API + admin UI button: POST /api/v1/terminals/orphans/cleanup with a dry-run flag.

Start by locating the reaper sweep and the Docker/Kubernetes workspace lifecycle paths. Review the GET /api/v1/auths/ user-list flow in JWT mode and the API-key-mode limitation, then clarify quarantine, retention, audit logging, and the optional POST /api/v1/terminals/orphans/cleanup endpoint. Done means orphaned directories and per-user PVCs are handled safely with recovery and audit evidence.