feat(billing): Stripe-backed billing engine (dark-launched)

Add a lazy/guarded Stripe client (boots without keys), Invoice/Payout schemas, per-currency Price.stripePriceIds, and a BillingService deriving partner/platform summaries, invoices and a partner-cut payout ledger. Partner and operator billing controllers plus a signature-verified Stripe webhook (Fastify raw body). Frontend: partner and operator billing pages and the operator tenant billing/audit tabs on real data. Gated behind new_billing_engine and BILLING_STRIPE_ENABLED; live money paths stay off until keys are set.

services/platform-api/src/main.ts

@@ -11,6 +11,9 @@ async function bootstrap() {
   const app = await NestFactory.create<NestFastifyApplication>(
     AppModule,
     new FastifyAdapter({ logger: true }),
+    // rawBody exposes req.rawBody (Buffer) — needed for Stripe webhook
+    // signature verification, which must hash the exact bytes Stripe sent.
+    { rawBody: true },
   )
 
   app.enableCors({