Add automated tests for backend templates

**Is your feature request related to a problem? Please describe.**
Currently, there are no tests or CI validation for backend apps. Basic smoke tests could be useful for validation

**Describe the solution you'd like**
Add basic tests via `pytest`/`fastapi.testclient` and add to CI for all template apps

**Additional context**
See https://fastapi.tiangolo.com/tutorial/testing/ for conventions
