feat: integrate platform backend and application interfaces

server/migrations/0015_v1_payment_request_fingerprints.sql

@@ -0,0 +1,6 @@
+-- Preserve payment/order request fingerprints across PostgreSQL reloads so
+-- idempotency checks cannot fall back to a lossy amount/plan comparison.
+ALTER TABLE IF EXISTS recharge_orders
+  ADD COLUMN IF NOT EXISTS request_fingerprint TEXT;
+ALTER TABLE IF EXISTS plan_purchases
+  ADD COLUMN IF NOT EXISTS request_fingerprint TEXT;