Allow event creators to enable on-site registration where attendees can register themselves at the event venue.

• Add checkbox "Enable on-site registration" to EventCreation form
• Store allow_onsite_registration boolean field in Event table (consider migrations correctly)
• Default to false for new events

• Update CreateEventRequest type to include allowOnsiteRegistration?: boolean
• Modify POST /api/events endpoint to accept and store new field
• Update Event interface to include allow_onsite_registration: boolean

• Add GET /api/events/{eventId}/signup endpoint returning event name and registration status
• Add POST /api/events/{eventId}/signup endpoint accepting { name: string }
• Only show registration form if allow_onsite_registration is true
• Create new attendee record with external_id set to "onsite-{timestamp}"

• Create OnSiteRegistration component with name input and submit button
• Display registration form on event page when enabled
• Show success message after registration
• Add validation for duplicate names (case-insensitive)

• Add allow_onsite_registration column to events table (default false) (consider migrations carefully)
• No changes to attendees or checkins tables required

• Use existing attendee flow for check-in after registration
• Leverage existing fuzzy search for finding registered attendees
• Maintain password protection for event management features