Increase the default ephemeral-storage request/limits

As per https://diamondlightsource.slack.com/archives/C051NDJH6BZ/p1786460035185709 pod sizes are getting towards the default size of ~2G. BlueAPI should set the request/limit in the default values.yaml template to be more than this so we have some more headroom. 4 or 8 is probably enough to last us a good while

## Acceptance Criteria
- The default BlueAPI deployment requests more storage

Start with the default values.yaml template and trace how its ephemeral-storage settings reach the BlueAPI deployment. Done means the default deployment requests and limits more storage than the current ~2G, with the rendered configuration reflecting the change.

Written by the indexing model from the issue text.