add API tests and mocked API tests

[markojakob]

No description provided.

[github-actions]

API & Mocked Tests and Coverage

Purpose: Validate README requirements for regular tests, mocked email tests, coverage, and runtime comparison.
Status: ❌ FAIL
Regular tests (app.test.js): ✅ PASS (37217 ms)
Mocked tests (app.mock.test.js): ✅ PASS (1164 ms)
Coverage gate (both runs at 100%): ❌ FAIL
Runtime gate (regular > mocked): ✅ PASS
Regular coverage: lines=100%, statements=97.05%, functions=100%, branches=94.44%
Mocked coverage: lines=81.81%, statements=79.41%, functions=80%, branches=77.77%
Run details: https://github.com/tanjaq/User-Register/actions/runs/23343872183
PR checks: https://github.com/tanjaq/User-Register/pull/24/checks
Where to look: Inspect the Run regular tests, Run mocked email tests, Enforce 100% coverage, and Enforce runtime rule logs in this run.