AES-256-GCM encryption for 18 PII fields across 6 tables (users,
guardians, identity_invites, space_invites, notifications, fund_claims).
HMAC-SHA256 hash indexes for email/UP address lookups. Keys derived from
JWT_SECRET via HKDF with dedicated salts. Dual-write to both plaintext
and _enc columns during transition; row mappers decrypt with plaintext
fallback. Includes idempotent backfill migration script.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>