Investigate runtime grey box testing (db queries/file system/security etc.)

Now that we have good black box coverage, I think it would be useful to introduce some grey box tests.
These would be against the full running system, but have access to some internal state.
Some things I am thinking of but we can expand:
- Query DB state, is everything as expected
- Query M/S/B filesystem
- Network traffic between services
etc.

Look at existing black box tests in the repository to understand the system's architecture. Identify entry points for monitoring database state, filesystem (M/S/B), and inter-service network traffic. Determine how to instrument the running system to expose internal state for verification. Define what 'done' looks like by creating a few example grey box tests that validate internal consistency.

Written by the indexing model from the issue text.