Fix user config cache expiry under load

[lucarlig]

Problem

lru_time_cache::LruCache refreshes an entry timestamp on access. The user-config cache used get_mut, so active subjects that received traffic more often than the configured expiry kept serving the original config indefinitely. Runtime-published Redis updates, including newly created virtual hosts, were not observed until traffic stopped long enough for the entry to expire or the process restarted.

Summary

• track user-config cache freshness from insertion time instead of LruCache access time
• refresh Redis config once nonzero cache expiry elapses even for hot subjects
• add unit coverage for insertion-time expiry and access not renewing freshness

Validation

• cargo fmt --check
• cargo test -p contextforge-gateway-rs-lib
• cargo clippy -p contextforge-gateway-rs-lib --all-targets -- -D warnings