feat(GRO-597): Stripe payment backend — schema, service, API, webhooks

Consolidates GRO-605, GRO-606, GRO-608 into a single clean PR:
- GRO-605: Stripe SDK integration + payment service
- GRO-606: Payment API endpoints (pay invoice, payment methods, refunds)
- GRO-608: Stripe webhook handler

Migration consolidation:
- Single 0026_stripe_payment.sql migration adds stripeCustomerId to clients
  and stripe_payment_intent_id, stripe_refund_id, payment_failure_reason to invoices
- Removed duplicate 0027_stripe_identifiers.sql

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/routes/invoices.ts

@@ -13,8 +13,9 @@ import {
   clients,
   sql,
 } from "@groombook/db";
+import type { AppEnv } from "../middleware/rbac.js";
 
-export const invoicesRouter = new Hono();
+export const invoicesRouter = new Hono<AppEnv>();
 
 const createInvoiceSchema = z.object({
   appointmentId: z.string().uuid().optional(),