feat(billing): sync catalog price edits to Stripe + re-price live customers

Editing a catalog amount now propagates beyond MongoDB. Stripe Prices are
immutable, so each changed currency mints a fresh Stripe Price at the new
amount, overwrites the cached stripePriceIds[currency] (which also fixes the
stale-price bug for new subscriptions), and repoints every live subscription
on that (row, currency) onto it with proration_behavior 'none' — the new
amount takes effect at the customer's next billing cycle, no mid-cycle charge.
The per-seat snapshot is refreshed so MRR reflects the go-forward rate.

Before committing the edit, the operator sees a warning with the affected
customer count, driven by a new GET /prices/:id/impact endpoint. Per-sub
failures are logged, never fatal; Stripe-disabled rows still re-snapshot.

services/platform-api/src/integrations/stripe.client.ts

@@ -114,6 +114,21 @@ export class StripeClient {
     await this.stripe.subscriptions.update(subscriptionId, { items: [{ id: itemId, quantity }] })
   }
 
+  // Repoint a subscription's line item at a different Price. Stripe Prices are
+  // immutable, so a catalog "price change" means minting a new Price and moving
+  // live subscriptions onto it. proration_behavior 'none' → no mid-cycle charge
+  // or credit; the new amount takes effect at the next renewal. Quantity is
+  // preserved (we only swap the price).
+  async updateSubscriptionPrice(subscriptionId: string, priceId: string): Promise<void> {
+    const sub = await this.stripe.subscriptions.retrieve(subscriptionId)
+    const itemId = sub.items.data[0]?.id
+    if (!itemId) throw new Error(`Stripe subscription ${subscriptionId} has no line items`)
+    await this.stripe.subscriptions.update(subscriptionId, {
+      items: [{ id: itemId, price: priceId }],
+      proration_behavior: 'none',
+    })
+  }
+
   async cancelSubscription(subscriptionId: string): Promise<void> {
     await this.stripe.subscriptions.cancel(subscriptionId)
   }