feat(GRO-608): Add Stripe webhook handler for payment events

- Add POST /api/webhooks/stripe endpoint for Stripe event processing
- Handle payment_intent.succeeded: mark invoice as paid, set paymentMethod=card
- Handle payment_intent.payment_failed: record failure reason on invoice
- Handle charge.refunded: mark invoice as void
- Handle charge.dispute.created: log dispute
- Idempotency: skip if stripePaymentIntentId already recorded
- Add stripe_payment_intent_id, stripe_refund_id, payment_failure_reason columns
- Requires STRIPE_WEBHOOK_SECRET env var for signature verification

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

apps/api/package.json

@@ -22,6 +22,7 @@
     "hono": "^4.6.17",
     "node-cron": "^3.0.3",
     "nodemailer": "^6.9.16",
+    "stripe": "^22.0.0",
     "zod": "^4.3.6"
   },
   "devDependencies": {