feat(billing): provision Stripe customer + subscription on tenant create

Wire the Stripe lifecycle into TenantsService (best-effort, gated on stripe.enabled): on create open a Stripe customer and, for a priced plan with seats >= 1, lazily create a Stripe Product+Price (persisted to Price.stripePriceIds[currency]) and a send_invoice subscription; mirror seat changes to the subscription quantity; pause/resume on suspend/resume; cancel on delete. A Stripe failure never blocks the tenant — the local Subscription stays the source of truth for derived MRR.

services/platform-api/src/prices/prices.service.ts

@@ -30,6 +30,19 @@ export class PricesService {
     return price.amounts[currency]
   }
 
+  // Persist the Stripe Price id we created for (this row, currency) so the
+  // next tenant on the same plan/cycle/currency reuses it instead of creating
+  // a duplicate Stripe Price. Keyed write — never clobbers other currencies.
+  async setStripePriceId(
+    priceId: PriceDocument['_id'],
+    currency: PriceCurrency,
+    stripePriceId: string,
+  ): Promise<void> {
+    await this.priceModel
+      .updateOne({ _id: priceId }, { $set: { [`stripePriceIds.${currency}`]: stripePriceId } })
+      .exec()
+  }
+
   async create(dto: CreatePriceDto): Promise<PriceDocument> {
     try {
       return await this.priceModel.create({ ...dto, active: dto.active ?? true })