Bulletproof retroactive wizard: fix all NOT NULL violations and edge cases

- spouses: add spouse_order, gender, classification, fallback date_of_birth
- children: add child_order, relationship, classification, fallback date_of_birth
- temporary_members: fallback date_of_birth, gender
- members: phone_mobile defaults to placeholder, date_of_birth fallback
- installment_plans: down_payment_receipt cast to string (VARCHAR column)
- All dates go through safeDate() — rejects garbage, parses valid formats
- All timestamps go through safeTimestamp() — never produces invalid datetime
- financial_year '2020/2021' extracted correctly for created_at
- bccomp amounts always cast to string to avoid type errors
- Null-safe on every optional field — zero room for SQL errors
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>