Self check-in for events and workshops

[mroderick]

Summary

Adds self check-in for event/workshop attendees. Organisers generate a landscape QR PDF, attendees scan it, authenticate with GitHub, and confirm their role. No admin intervention needed at the door.

Key changes

• check_in_code column on events/workshops: 3-word EFF wordlist code (e.g. salon-grew-sprang) used in the check-in URL path (/check-in/e/:code / /check-in/w/:code)
• source column on invitations/workshop_invitations: tracks how an invitation was created — email, check_in, or admin (nullable, no default)
• CheckInCode concern: lazy-loaded EFF wordlist (7,776 words), generates unique codes on demand
• CheckInPdf service: Prawn-generated landscape A4 PDF with codebar SVG mark, event/workshop details, venue, sponsors, and QR code
• Admin::CheckInsController: instructions page with PDF download button; polymorphic routing under events/workshops
• Public CheckInsController: GitHub auth redirect, role selection (Student/Coach with inference from existing invitations/group subscriptions), confirmation page
• Check-in button on admin event/workshop toolbar linking to the instructions page

Notes

• Events set verified=true on check-in; workshops set attended=true
• No backfill rake task for check_in_code — generated lazily on first PDF download
• Uses prawn-svg gem (pure Ruby SVG renderer, no native deps)
• The revert commit (70343ed) was a UX iteration that restored the 120pt logo mark size; this whole chain can be squashed into clean commits