The login rate limiter keyed on the client-controlled x-forwarded-for
header, letting an attacker rotate XFF for unlimited password guesses
while also growing the failures Map unboundedly (memory DoS). Since
this is a single-password instance, replace with one global
{count, resetAt} tracker per app instance: check the 15-min window and
reject at >=10 failures before verifying the password, increment on
failure, reset on success.
