feat: implement complex user invitation handler with tests and configuration

apps/backend/README.md

@@ -9,3 +9,21 @@ bun run dev
 ```
 
 open http://localhost:3000
+
+## Tests
+
+Run all backend tests:
+```sh
+bun run test
+```
+
+### Test naming convention
+
+- `*.test.ts` or `*.spec.ts`: service-level tests. These run with the shared Prisma preload at `test/support/prisma.mock.ts`.
+- `*.handler.test.ts`: handler-level tests. These should mock the service module locally with `mock.module(...)` and import the handler after the mock is in place.
+
+### Recommended pattern
+
+- Keep business logic tests focused on the service layer.
+- Use handler tests only to verify request parsing, status mapping, and response shaping.
+- Put shared Prisma mocking helpers in `test/support/`.