Add a CLIENT_IP_HEADERS escape hatch and stop the auth handler hanging

Two changes, both from working out how to verify the client-IP configuration
on the Plesk vhost before deploying.

CLIENT_IP_HEADERS=none now trusts no header at all. This covers the one case
where the wrong setting is worse than no setting: if the proxy does not
overwrite the header we trust, any client can send it and mint itself an
unlimited brute-force budget against /sign-in. Falling back to the shared
rate-limit bucket is a nuisance - one noisy client can lock everyone out for
ten seconds at a time - but it fails closed, and it can be reverted from the
environment without a deploy. An empty or unset value still means "use the
default": a stray blank line in a .env must not silently change how requests
are bucketed, so only the explicit word does that. An empty array is what
better-auth reads as "no headers"; it falls back to its own default only when
the option is absent, and [] is truthy.

The TRUSTED_PROXY_IPS boot warning was overstating the risk. It now says that
a single-value header needs no trusted proxies, so seeing it on a plain
single-proxy setup is expected rather than a problem to chase.

Separately: a rejected promise from better-auth's handler used to escape as an
unhandled rejection, leaving the request hanging forever with no response while
the process logged an uncaughtException. Express 4 does not await an async
handler, and nearly every better-auth route touches the admin database, so any
database blip would have done this. Found by pointing ADMIN_DB at a database
the user cannot open while testing the hatch. The handler now answers 503, so
the caller learns and the other domains keep serving; verified as a 24ms
response instead of a hang, with / and /feedback/admin/me unaffected.

test/admin/admin.config.test.ts is new: it pins both branches of the hatch and
the rule that an unset NODE_ENV counts as production. It stubs dotenv, because
admin.config would otherwise read the repo's own .env and quietly reintroduce
NODE_ENV=development - the exact value several of those cases exist to remove.

157 unit tests and 41 integration tests pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/models/admin/admin.config.ts

@@ -101,16 +101,53 @@ export const ADMIN_ALLOWED_ORIGINS = Array.from(new Set([
  * brute-force budget - so the default is the single header nginx sets, not a
  * permissive list.
  */
-export const CLIENT_IP_HEADERS = parseList(process.env.CLIENT_IP_HEADERS, ['x-real-ip']);
+
+/**
+ * `CLIENT_IP_HEADERS=none` trusts no header at all.
+ *
+ * This is the escape hatch for the one case where the wrong setting is worse
+ * than no setting: if the proxy turns out NOT to overwrite the header we are
+ * trusting, any client can send it and mint itself an unlimited brute-force
+ * budget against /sign-in. Falling back to the shared bucket is bad (one noisy
+ * client can lock the organisation out for ten seconds at a time) but it is
+ * bad in a way that fails closed, and it can be reverted from the environment
+ * without a deploy.
+ *
+ * Reach for it only after a check has actually failed - `SELECT ipAddress FROM
+ * session ORDER BY createdAt DESC` showing 127.0.0.1 or NULL for a real remote
+ * sign-in - and take it back out once the header is configured.
+ *
+ * An empty or unset value still means "use the default", not "trust nothing":
+ * a stray blank line in a .env must not silently change how requests are
+ * bucketed. Only the explicit word does that.
+ */
+const TRUST_NO_HEADER = 'none';
+
+export const TRUST_NO_CLIENT_IP_HEADER =
+	(process.env.CLIENT_IP_HEADERS || '').trim().toLowerCase() === TRUST_NO_HEADER;
+
+// An empty array is what better-auth reads as "no headers": it only falls back
+// to its own default when the option is absent, and `[]` is truthy.
+export const CLIENT_IP_HEADERS = TRUST_NO_CLIENT_IP_HEADER
+	? []
+	: parseList(process.env.CLIENT_IP_HEADERS, ['x-real-ip']);
 
 export const TRUSTED_PROXY_IPS = parseList(process.env.TRUSTED_PROXY_IPS, []);
 
-if (isProd && TRUSTED_PROXY_IPS.length === 0) {
+if (isProd && TRUST_NO_CLIENT_IP_HEADER) {
+	logger.warn(
+		'Admin module: CLIENT_IP_HEADERS=none - no client-IP header is trusted, so every ' +
+		'request shares one rate-limit bucket and /sign-in allows 3 attempts per 10 seconds ' +
+		'for everyone combined. This is the safe fallback, not a destination: configure the ' +
+		'header the proxy actually sets and remove it.'
+	);
+} else if (isProd && TRUSTED_PROXY_IPS.length === 0) {
 	logger.warn(
 		'Admin module: TRUSTED_PROXY_IPS is not set. If the proxy sends a multi-value ' +
 		`${CLIENT_IP_HEADERS.join('/')}, better-auth cannot resolve a client IP and every ` +
 		'request shares one rate-limit bucket. Verify with: SELECT `key` FROM rateLimit - ' +
-		'a "no-trusted-ip" row means this is happening.'
+		'a "no-trusted-ip" row means this is happening. A single-value header needs no ' +
+		'trusted proxies, so this warning is expected on a plain single-proxy setup.'
 	);
 }