DEICH-5829 Error handling, retry logic and refactoring (see details)

Many small things to tie everything together. See individual commits for more details.

Excerpt of changes:

• remove the boostrapper/startup combo and use the preload technique from dotenv

• introduce a env-util that loads INTERNAL_URL_* or EXTERNAL_URL_* based on the context

• extract logger code to logger.js

• add and fix some tests (still WIP)

• start refactoring process.js (still WIP)

• add a User Agent when requesting images from Discogs (part of TOS)

• improve metadata.js and make it work with Discogs

• add and use PQueue in processor.js (makes it easier to know when things are done)

• extract allowed file types to constants

• improve mimir.js

• make Euler refresh tokens in a better way

• Prometheus additions was merged into this branch

Closes DEICH-5829