v1.5.3

Added

• JwksClient.prefetch!(url) — fire-and-forget JWKS key fetch in background thread
• JwksClient.start_background_refresh!(url, interval:) — Concurrent::TimerTask for hourly key refresh
• JwksClient.stop_background_refresh! — stops background refresh timer task
• bootstrap_lease_ttl: 300 in vault defaults (5-minute TTL for bootstrap credentials)

Changed

• JwksClient.clear_cache now also stops any running background refresh task