Implement EAV system for dynamic event-specific registration fields
with organisation-level templates, person section preferences with
priority ranking, and TagSyncService for deferred tag_picker sync.
New tables: registration_field_templates, registration_form_fields,
person_field_values, person_section_preferences.
New columns: persons.remarks, events.registration_show_section_preferences,
events.registration_show_availability.
58 tests, 126 assertions — all 432 tests pass (zero regressions).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
