feat(billing): scaffold billing module with Stripe SDK

Add billing module structure following existing module patterns
(controllers, services, routes, models, policies, middlewares,
repositories, tests) with development config for Stripe integration.

Closes #3240

Author: PierreBrisorgueil

modules/billing/config/billing.development.config.js

@@ -0,0 +1,18 @@
+const config = {
+  stripe: {
+    secretKey: 'sk_test_placeholder',
+    webhookSecret: 'whsec_placeholder',
+    prices: {
+      starter: {
+        monthly: 'price_starter_monthly_placeholder',
+        annual: 'price_starter_annual_placeholder',
+      },
+      pro: {
+        monthly: 'price_pro_monthly_placeholder',
+        annual: 'price_pro_annual_placeholder',
+      },
+    },
+  },
+};
+
+export default config;