fix: add periodic cleanup of login_attempts hash

Sweep stale entries every 5 minutes to prevent unbounded memory growth
from IPs that touch the login endpoint once and never return.

lib/volumen/server.rb

@@ -224,6 +224,7 @@ module Volumen
     end
 
     def check_login_rate_limit!
+      Volumen.sweep_login_attempts!
       ip = request.ip.to_s
       now = Time.now.to_f
       cutoff = now - LOGIN_WINDOW