Error saving cache

## Description

I found this error in the Harmony prod server logs. Not sure exactly which part of the code base it originates from.

```
INFO: Cache instruments_cache.json saved...
Could not save vectors cache: Object of type ndarray is not JSON serializable.
```

## Environment

Centos

## How to Reproduce

1. Run Harmony in Docker `docker run -p 8000:8000 harmonydata/harmonyapi:20250311105945_1df2e89`
2. Run any harmonisation
3.

## Expected Behaviour

The cache should be saved without errors. It looks like we're trying to serialise a Numpy array to JSON. Maybe you can find that part of the code and switch it to Pickle?