Use real respondents in emailed survey demos

[ttenneb]

Summary

• make emailed demos use the survey's actual respondent-backed tagbox choices
• preserve the configured survey schema instead of substituting synthetic demo values
• continue validating signed demo links and active surveys without recording demo responses
• warn senders that demo links contain real respondent names and email addresses

Testing

• npm --prefix api test (55 passed)
• npm --prefix dashboard test (47 passed)
• npm --prefix dashboard run build
• git diff --check