feat(11-01): add PAX8 subscription cost columns (migration 092)

- ALTER TABLE pax8_subscriptions to add price/partner_cost/currency
- Additive, idempotent (IF NOT EXISTS), matches currency convention from 091
- Applied to dev DB via docker exec (Postgres init won't re-run on existing volume)

migrations/092_pax8_subscription_costs.sql

@@ -0,0 +1,20 @@
+-- PAX8 subscription cost columns.
+--
+-- PAX8's Subscription object (GET /subscriptions?page=&size=, Phase 11
+-- research, devx.pax8.com findsubscriptions) exposes both a customer/list
+-- `price` and a partner/reseller `partnerCost` per subscription, plus a
+-- `currencyCode`. Migration 091 did not model these — this migration adds
+-- them as additive columns on the existing pax8_subscriptions table (D-03).
+--
+-- Raw per-billing-period amounts are stored exactly as PAX8 returns them,
+-- paired with the existing billing_term column. Monthly normalization is a
+-- read-time concern, not modeled here (D-04).
+
+ALTER TABLE pax8_subscriptions
+  ADD COLUMN IF NOT EXISTS price NUMERIC(12,2);
+
+ALTER TABLE pax8_subscriptions
+  ADD COLUMN IF NOT EXISTS partner_cost NUMERIC(12,2);
+
+ALTER TABLE pax8_subscriptions
+  ADD COLUMN IF NOT EXISTS currency CHAR(3) NOT NULL DEFAULT 'USD';