Root cause: after WaiverProcessor::execute() completed, visiting /members/{id}
triggered MembershipPaymentGuard::reconcile(), which did not recognise waiver_fee
as a valid activation payment. It stripped the membership_number and then called
deactivateAllDependents(), which crashed on spouses.join_date NOT NULL constraint.

Fixes:
- MembershipPaymentGuard::reconcile(): add waiver_fee path — looks up completed
  waiver_requests where target_member_id matches, preventing false deactivation
- MembershipPaymentGuard::deactivateAllDependents(): spouses.join_date is NOT NULL;
  use sentinel date '1970-01-01' instead of NULL to avoid constraint violation
- MembershipPaymentGuard::deactivateDependent(): same sentinel fix for spouses
- WaiverProcessor::execute(): set activated_by_payment_id + activated_at on the
  target member so the existing fallback check in reconcile() catches future cases

Also restored member #136 directly in DB (membership_number='1600', status='active',
activated_by_payment_id=773) which was the live victim of this bug.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>