Rename customer_* columns on ga_visitors to person_*.

The original customer_ prefix prejudged the host's relationship type and overlapped semantically with the customer value in the status enum. The neutral person_ prefix accurately describes these as the host-app's projection of who the visitor is, regardless of lifecycle stage.

Columns renamed:

• ga_visitors.customer_external_id → person_external_id
• ga_visitors.customer_email → person_email
• ga_visitors.customer_name → person_name
• ga_visitors.customer_metadata → person_metadata

Indexes renamed:

• idx_ga_visitors_customer → idx_ga_visitors_person_external_id
• idx_ga_visitors_email → idx_ga_visitors_person_email

Renames take a brief AccessExclusiveLock per ALTER but no data movement (PostgreSQL renames are catalog-only).