Add admin-assisted password reset

There's no email infrastructure in this app, so a self-service
"forgot password" flow isn't feasible yet. Adds a "Reset Password"
button per user in the admin panel instead: generates a random
temporary password (shown once, for the admin to relay out-of-band),
overwrites the user's password hash, and signs them out of every
existing session so a stolen session can't outlive the reset.

Verified live: old password rejected after reset, new temporary
password logs in successfully.

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

server/auth.js

@@ -87,3 +87,8 @@ export async function consumeBackupCode(submitted, hashes) {
   }
   return null;
 }
+
+/** A random temporary password for admin-assisted resets — shown once, relayed to the user out-of-band. */
+export function generateTemporaryPassword() {
+  return randomBytes(6).toString('hex');
+}