The admin panel already checked CSRF tokens on destructive actions,
but the player-facing raw HTML forms (create/join/leave/start
session, toggle ready, lobby chat, feedback) had none - cookie
SameSite=Lax blunts classic cross-site auto-submit attacks but isn't
a substitute for real tokens. Adds matching csrf_token()/
isCsrfTokenValid() checks to all of them.
Also adds login_throttling (5 attempts/15 min) to stop unlimited
password guessing, and a per-user rate limiter (10/min) on the
invite-code join endpoint, since invite codes are only 32-bit and
had no protection against brute-forcing.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
