- Add single-retrieval semantics: approved PKI bundles are atomically
removed from the in-memory cache on first retrieval via DashMap::remove(),
preventing concurrent requests from obtaining the private key
- Add TTL expiry: ApprovedEntry wraps PkiBundle with approved_at and ttl
fields; bundles expire after ENROLLMENT_BUNDLE_TTL_SECS (600s / 10 min)
- Replace brute-force clear() purge with TTL-based retain() in background
task, running every 60s instead of every 600s
- Audit tracing calls: confirm no raw polling token is logged; add security
comment documenting this policy
- Document CSR-based enrollment as future enhancement in both enrollment.rs
and SECURITY.md, explaining why server-generated keys are used currently
