Two fixes:
1. handleRegistrationFormPaid now also sets registration_fee_paid=1 on
   sa_players, so next time the system knows they already paid.
2. lookupPlayer checks for existing in-progress/completed registration
   and redirects to it instead of creating a duplicate. This means a
   returning player who already completed all steps goes back to step 4
   (their finished registration), not a new step 1.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>