chore: paralleltest

[FoseFx]

Which issues does this pull request close?

Re: #865

Enables paralleltest linter, by calling t.Parallel() for every test, for this:

• remove global state for db and otel Registry (now injected into start-up context)
• add middlewares to propagate these values into the request contexts
• rename GetDB to MustGetDB, and require context as parameter
• add AddUserToOrganization (to unify a common pattern in test code into a proper function)

This should also enable test code to create other services than the own.