Security improvements

### User Story

As a user of Sleeper, I want issues with security implications to be dealt with quickly, so that I can be confident that all potential security problems have been addressed.

### Description / Background

We'd like to ensure that Sleeper has an appropriate level of security, and that we implement security measures that we can easily take, or that can make a significant impact.

We'd like to ensure that appropriate automated vulnerability scanning tools are in place, and we address issues found by these promptly.

### Technical Notes / Implementation Details

See sub-issues. We may make notes here on potential issues that have not yet been raised.

Start by reviewing the sub-issues referenced in this umbrella issue and identify the existing automated vulnerability-scanning setup. The work is complete when the agreed security measures are implemented and vulnerability findings are addressed promptly, but this issue alone does not specify individual files, tests, or acceptance criteria.

Written by the indexing model from the issue text.