Cached metadata is now served directly within a configurable TTL window
(default 5m) without contacting upstream, reducing latency and upstream
load. When upstream is unreachable and the cache is past its TTL, stale
content is served with a Warning: 110 header per RFC 7234.
New config: `metadata_ttl` (YAML) / `PROXY_METADATA_TTL` (env).
Set to "0" to always revalidate with upstream.