Replace thread-ID-based conversation matching with email-address-based
matching for more reliable conversation continuity. Key changes:
- One conversation per email address (simpler model)
- No data expiration (conversations persist indefinitely)
- Post-completion support (questions and registration updates)
- Versioned storage for registration updates (audit trail)
- Admin notifications for registration changes
This addresses the gap where parents sending new emails (instead of
replying) would lose their registration progress.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
