Post-event survey: reminder/nudge emails for incomplete surveys

🤖 From Claude: Filed while spiking the post-event survey feature (deferred from that PR).

Context

The post-event survey feature delivers survey forms inline on the registration ticket (Day 1 / Day 2 / Scholarship-recipient surveys), tracks completion, and sends an FYI email on submission. It does not nudge registrants who haven't completed a survey.

What's wanted

Reminder/nudge emails to registrants with an outstanding, past-drip, required survey — e.g. a scheduled reminder after the drip date, and/or a manual "send reminder" admin action from the registrants roster.

Notes

• Reuse the readiness survey_pending signal to find who's outstanding.
• Respect the survey's drip date (don't nudge before it's available) and the per-registrant applicability (day surveys for attendees, scholarship survey for recipients).
• Follow existing mailer + preview conventions.

Start with the readiness survey_pending signal and the existing mailer and preview conventions. Trace how drip dates and per-registrant applicability are represented, then inspect the registrants roster entry point for a possible manual action. Done means eligible registrants with past-due incomplete surveys can receive an appropriate reminder without nudging ineligible or not-yet-available surveys.