Add generate_*_id() functions for consistent short ID generation across all tables.
- Convert user_social_logins, verification_codes, custom_services,
time_blockers, forgiven_no_shows, user_referrals from BIGSERIAL to CHAR(12)
- Add name_history, referral_discounts tables
- Add dedicated generate_*_id() functions for each table
- Add FK from name_history.booking_id to bookings(id)
- Add referral_discounts index on user_id
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>