✨ add GDPR consent field to event registration

[solarmarius]

Changes:

• Add gdprConsent to JoinEventPayload and Participant models (both optional)
• Update join_event endpoint to capture gdprConsent value
• Update get_event_options to return gdprConsent with safe .get() for legacy data
• Include gdprConsent in event export with summary statistics
• Update test fixtures and seeding script to include gdprConsent

Backward compatibility:

• gdprConsent defaults to None for existing participants