feat(security): hook-event auth secret + tunnel password guard

Two hardening fixes for the public-tunnel exposure path (COD-54 / COD-55).

COD-54 — gate the /api/hook-event localhost bypass when a tunnel is up:
`cloudflared --url http://127.0.0.1:port` proxies internet traffic INTO the
loopback origin, so a tunneled hook request arrives with req.ip === 127.0.0.1
and the old bare-localhost bypass would pass it unauthenticated. Now:
- tunnel running  → bypass requires a shared per-instance hook secret
  (X-Codeman-Hook-Secret header; constant-time compare) + per-IP rate limiting
- tunnel not running (loopback-only, the normal case) → unchanged, so
  already-deployed credential-less hooks keep working.
New src/config/hook-secret.ts; auth middleware takes a getTunnelRunning probe
(wired from server.ts via tunnelManager.isRunning()).

COD-55 — refuse starting the Cloudflare tunnel without auth:
enabling the tunnel publishes full terminal control to a public URL; with no
CODEMAN_PASSWORD the auth middleware is inactive and the bind guard never trips
(tunnel binds loopback). PUT /api/settings now refuses tunnelEnabled:true with a
403 (before persisting) unless CODEMAN_PASSWORD is set or
CODEMAN_ALLOW_UNAUTHENTICATED_NETWORK=1 is acknowledged. New
isUnauthenticatedNetworkAcknowledged() in network-auth-policy; settings-ui
surfaces the refusal as an error toast and reverts the toggle.

Scope: the always-on CSRF/Origin guard, Host-header allowlist, and
network-auth-policy itself are already upstream (#113) and not re-proposed here.

Verification: tsc, eslint, prettier, check:frontend-syntax clean; full test:ci
green (2723 passed), incl. test/cod54-hook-event-auth and
test/routes/system-routes-tunnel-guard.

src/web/network-auth-policy.ts

@@ -6,6 +6,16 @@ export function isExplicitlyEnabled(value: string | undefined): boolean {
   return value !== undefined && EXPLICIT_TRUE_VALUES.has(value.trim().toLowerCase());
 }
 
+/**
+ * True when unauthenticated network exposure is acceptable: either a password is
+ * set (auth active) or the operator explicitly acknowledged it. Used by the
+ * tunnel-enable guard (COD-55) to refuse publishing an unauthenticated public URL.
+ */
+export function isUnauthenticatedNetworkAcknowledged(allowFlag = false): boolean {
+  if (process.env.CODEMAN_PASSWORD) return true;
+  return allowFlag || isExplicitlyEnabled(process.env.CODEMAN_ALLOW_UNAUTHENTICATED_NETWORK);
+}
+
 export function isLoopbackBindHost(host: string): boolean {
   const normalized = host
     .trim()