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

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

apps/api/tests/providers/registry.test.js

@@ -0,0 +1,51 @@
+'use strict';
+
+// ── Provider contract test ────────────────────────────────────────────────────
+// This suite is the "gate" for onboarding new payment providers. It does not
+// hardcode "paypal" as the only case — it walks registry.list() and asserts
+// every registered provider satisfies the shape payment.service.js relies on.
+// Drop a second provider into providers/registry.js and this file validates it
+// for free, with zero new test code required.
+
+const registry = require('../../providers/registry');
+
+const REQUIRED_METHODS = ['createOrder', 'captureOrder', 'refundCapture'];
+
+describe('payment provider registry', () => {
+
+  test('lists at least one provider', () => {
+    expect(registry.list().length).toBeGreaterThan(0);
+  });
+
+  test('currently registers paypal', () => {
+    expect(registry.list()).toContain('paypal');
+  });
+
+  test('get() returns the provider module for a known name', () => {
+    const provider = registry.get('paypal');
+    expect(provider).toBeDefined();
+  });
+
+  test('get() throws a descriptive error for an unknown provider', () => {
+    expect(() => registry.get('stripe')).toThrow(/Unknown payment provider: "stripe"/);
+  });
+
+  test('unknown-provider error lists the available providers so misconfiguration is easy to diagnose', () => {
+    try {
+      registry.get('does-not-exist');
+      throw new Error('expected registry.get to throw');
+    } catch (err) {
+      registry.list().forEach((name) => {
+        expect(err.message).toContain(name);
+      });
+    }
+  });
+
+  describe.each(registry.list())('provider contract: %s', (name) => {
+    const provider = registry.get(name);
+
+    test.each(REQUIRED_METHODS)('exposes %s as a function', (method) => {
+      expect(typeof provider[method]).toBe('function');
+    });
+  });
+});