docs(511): add payout clawback before/after verification screenshots

Live `/platform/payouts` captures from the issue's worked example (Default
School at 80%, 1000 successful + 100 refunded, 80 paid out): the balance
reads 640 on master and 720 with the fix, and 800 when no payout was ever
recorded.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012nmbfroRHjSNrkGbRqTozs

Author: guillermoscript