use main.py instead of app.py as mainfile

[TheCataliasTNT2k]

Describe the bug
The microservice was initialized twice, because Python renames the entrypoint internally to __main__. We have a circular import back to the entrypoint app, which then initializes the microservice again. That caused many problems.

Solution
Move the main entrypoint from app.py to a new file, e.g. main.py.