refactor: Make inventory class-based

[RDIL]

Scope

Two big changes here:

1. inventory is a class now, so the stateful parts are not stored on a module level
2. controller instance can be overridden for tests, so that code which hasn't yet been migrated to the IoC model can use the intended controller instance.

Test Plan

• Start the server, verify that logging in still works

Checklist

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General

• I've run Prettier to format any changed files
• I've verified that my changes work, and included a test plan

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Testing

• I have added or considered adding unit/integration tests that cover any code changes

[alex73630]

Overall LGTM

Of course it would be better to have it checked against tests but I know that this change is required to get tests running.