fix(video): re-anchor transcript log-redaction so PII/credential maskers can't reopen the leak (#12150 P1 follow-up) (#12503)

* fix(video): re-anchor log-redaction fullText from the finished guardrail payload so PII/credential maskers can't reopen the transcript leak

* docs(video): clarify P1 transcript-retention scope and re-anchor; list P2 surfaces (#12430)