chore: relocate backend into apps/api ahead of monorepo merge

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/api/providers/registry.js

@@ -0,0 +1,14 @@
+'use strict';
+
+const paypal = require('./paypal.provider');
+
+const PROVIDERS = { paypal };
+
+module.exports = {
+  get(name) {
+    const p = PROVIDERS[name];
+    if (!p) throw new Error(`Unknown payment provider: "${name}". Available: ${Object.keys(PROVIDERS).join(', ')}`);
+    return p;
+  },
+  list: () => Object.keys(PROVIDERS),
+};