fix(auth): strengthen admin temp-password entropy to 128 bits

generateTempPassword() used randomBytes(4) — 32 bits behind a known
'Bifrost-' prefix — for a directly-usable login password set by the admin
reset action. Brute-forceable. Bump to randomBytes(16) (128 bits) base64url.
Flagged by automated security review.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/auth.ts

@@ -23,9 +23,11 @@ export function verifyPassword(password: string, hash: string): boolean {
 }
 
 /** A readable one-time password for admin resets. Give to the user; they
- *  change it from /account. */
+ *  change it from /account. 16 bytes = 128 bits of entropy from the CSPRNG
+ *  (the 'Bifrost-' prefix is fixed/known, so the randomness must carry the
+ *  full strength on its own). */
 export function generateTempPassword(): string {
-  return 'Bifrost-' + randomBytes(4).toString('hex');
+  return 'Bifrost-' + randomBytes(16).toString('base64url');
 }
 
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