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/index.ts

@@ -28,6 +28,7 @@ import { resolveStaffMiddleware, requireRole, requireRoleOrSuperUser, requireSup
 import { devRouter } from "./routes/dev.js";
 import { adminSeedRouter } from "./routes/admin/seed.js";
 import { startReminderScheduler } from "./services/reminders.js";
+import { webhooksRouter } from "./routes/stripe-webhooks.js";
 
 const app = new Hono();
 
@@ -50,6 +51,9 @@ app.route("/api/book", bookRouter);
 // Public portal routes — client-facing, authenticated via impersonation session header
 app.route("/api/portal", portalRouter);
 
+// Public Stripe webhook endpoint — signature-verified, no auth required
+app.route("/api/webhooks/stripe", webhooksRouter);
+
 // Dev/demo routes — config is always public, users endpoint is guarded internally
 app.route("/api/dev", devRouter);