feat(schema): add check-in and external attendance fields

[AlexVOiceover]

Summary

• Add Public checkbox and Check-in Code fields to Events table in Airtable
• Add External Name and External Email fields to Attendance table
• Update TypeScript types and events client to support new fields
• Update tests and schema documentation

Changes

• Airtable schema: Added Public (checkbox), Check-in Code (number) to Events; External Name/Email to Attendance
• config.ts: Added new field IDs for all new fields
• event.ts types: Added isPublic: boolean and checkInCode?: number to Event interface
• events.ts client: Updated listEvents, getEvent, createEvent, updateEvent to map new fields
• events.spec.ts: Updated test expectations to include new fields
• schema.md: Documented new fields

Test plan

• All existing tests pass with new field expectations
• TypeScript check passes
• ESLint passes

Related

Closes AP-19