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/app.module.ts

@@ -3,6 +3,7 @@ import { ConfigModule } from '@nestjs/config'
 import { MongooseModule } from '@nestjs/mongoose'
 import { AuditModule } from './audit/audit.module.js'
 import { AuthModule } from './auth/auth.module.js'
+import { BillingModule } from './billing/billing.module.js'
 import { FlagsModule } from './flags/flags.module.js'
 import { HealthModule } from './health/health.module.js'
 import { IngestModule } from './ingest/ingest.module.js'
@@ -30,6 +31,7 @@ import { UsersModule } from './users/users.module.js'
     SubscriptionsModule,
     PricesModule,
     FlagsModule,
+    BillingModule,
     IngestModule,
     SeedModule,
   ],