docs(514): add billing-health at-risk coverage before/after captures

Live `/platform/billing-health` captures against five seeded fixtures (a
lapsed cutoff-only school, an already-paused one, a subscription-only
past-due school, a manual-transfer school two days from downgrade, and a
Stripe dunning one):

- master lists only the two `billing_status='past_due'` schools.
- PR #523's union lists all five, but the already-downgraded school reports
  "Past due since Jun 10" with a red "Overdue" and sorts above the school
  that actually downgrades in two days.
- The follow-up gates those columns on the tenant being past due, so the
  live countdowns come back to the top.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WJDnXnWmdBDTX33bvTx62K

Author: guillermoscript