Add Stripe payment integration and update configuration

- Added Stripe environment variables to docker-compose.yml for secret key, webhook secret, and base URL.
- Created PAYMENTS.md to document the setup and usage of the Stripe payment system.
- Updated webhook signature handling in stripe.js to use a new secret key.

Signed-off-by: Omar Sánchez Pizarro <omar.sanchez@pistacero.net>

web/backend/services/stripe.js

@@ -214,7 +214,7 @@ export function verifyWebhookSignature(payload, signature) {
   let webhookSecret = process.env.STRIPE_WEBHOOK_SECRET;
 
   if (!webhookSecret) {
-    webhookSecret = 'whsec_8ebec8c2aa82a791aa9f2cd68211e297a5d172aea62ebd7b771d230e3a597aa8';
+    webhookSecret = 'whsec_n6tTKRSG38WJQDRX8jLjZTs7kPKxbdNP';
   }
 
   if (!webhookSecret) {