docs(billing): record OIG/SAM recurring validation status

Checkout half proven against live Stripe (dry-run session created + expired,
zero charge), webhook subscription-id extraction + worker renewal fulfillment
covered by unit tests (31 + 13). Remaining gap: full E2E with a Stripe test
clock, which needs test-mode keys in the server .env (currently unset).

docs/billing.md

@@ -235,6 +235,29 @@ screening; customers are converted to the $79/mo monitoring subscription after
 that first cycle (the standalone $948/yr of monitoring is no longer given away
 inside the bundle).
 
+**Validation status (2026-06-18):**
+- ✅ *Checkout half — proven against LIVE Stripe.* A dry-run created a real
+  `mode:subscription` Checkout Session with the exact production params
+  (recurring `price_data`, `unit_amount=7900`, `recurring.interval=month`,
+  `card`+`us_bank_account`, metadata) — Stripe returned `amount_total=7900`,
+  `type=recurring`, then the session was immediately **expired** (creating a
+  session never charges anyone; only a completed hosted page does). Net effect
+  on prod: zero.
+- ✅ *Webhook subscription-id extraction* — `invoiceSubscriptionId()` unit tests
+  (31 in `api/tests/recurring-subscription.test.ts`) cover acacia (top-level)
+  AND dahlia (nested) invoice shapes, renewal-cycle gating, surcharge
+  suppression, recurring line-item build.
+- ✅ *Worker renewal fulfillment* — `scripts/workers/services/test_npi_recurring.py`
+  (13 assertions) runs the real handler and asserts the `[Monthly cycle]` /
+  re-screen behaviour; passes locally + in the deployed workers container.
+- ⏳ *Full end-to-end with a Stripe test clock* — NOT yet run. Requires
+  `STRIPE_TEST_SECRET_KEY` / `STRIPE_TEST_WEBHOOK_SECRET` in the server `.env`
+  (currently unset; prod is `NODE_ENV=production`). Once those exist: place a
+  test-card subscription, advance a billing cycle via a test clock, and confirm
+  the live `invoice.paid` (subscription_cycle) re-dispatches the screening
+  worker and a fresh certificate is issued. This is the last gap before a real
+  recurring charge should be marketed.
+
 ### Formation Maintenance Bundles
 
 | Bundle | Price | Includes | Savings |