fix: fail closed on an unwritable audit log, bound the rate limiter and harden startup

The audit logger is opened at startup (exit 2 on failure) and obtained before
every call, which is refused with audit_unavailable otherwise. Audit records only
take validated path and sudo values and use the request id as correlation id. The
rate limiter evicts idle keys, caps tracked keys, audits the first excess per key
and window and uses the settings passed to the app. FORWARDED_ALLOW_IPS is now a
setting, and startup warns about a missing host allow-list and unavailable OAuth.

docs/hardening.md

@@ -40,6 +40,7 @@ authentication), but it does not decide what is safe. This checklist is how you
   rejected.
 - Restrict who can reach the endpoint at the proxy or firewall too (VPN, allow list), as a
   second layer in front of bearer authentication.
+- Set `FORWARDED_ALLOW_IPS` to the proxy address only (not `*`), so a client cannot spoof its IP.
 - Leave `RATE_LIMIT_PER_MINUTE` on. Put request-size and connection limits at the proxy.
 - Use https for `GITEA_URL`. Plain http sends the token unencrypted (setup warns about it).