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?

Reproduce the failure with the documented Docker command and search the codebase for "Could not save vectors cache" or the vectors cache handling. Confirm how the ndarray is serialized and verify that a harmonisation run saves the cache without the JSON serialization error.

Scritto dal modello di indicizzazione a partire dal testo della issue.