Two issues flagged by argus in PR #2388 review:
1. Empty-key fallback: when load_or_generate() failed, webhook_secret was
String::new(). An attacker knowing this could forge deliveries with a
valid HMAC of the empty key. Fix: change to Option<String>; on None,
skip hook registration entirely and return 503 from /webhook/* handlers
(rather than 401 with a misleadingly-verifiable empty-key HMAC).
2. Stale hook cleanup: on upgrade from old code, old loopback hooks
(http://127.0.0.1:.../webhook/knowledge, .../webhook/config-pr) were
left alongside the new domain-URL hook. Fix: during ensure_webhook /
ensure_config_pr_webhook, after listing hooks, delete any that end with
our path suffix but point at a different base URL.
clippy + nix fmt clean.
7b1b1d9db8