Slack display names in the API

This will make things easier for people consuming the API who want to show names (without extra calls to Slack or cachet.dunkirk.sh)

This will also require a periodic job to sync Slack display names in the DB with Slack (or it could listen for events). We should then apply that to usernames too since those can technically change.

### Links

- [Feature request on Slack](https://hackclub.slack.com/archives/C09QR2BH3GE/p1783442855889219)

Start by locating the API responses for users and the database fields that store Slack identities. Then investigate how this service could periodically sync Slack display names, or consume Slack events, and how the same approach would apply to usernames. Done means API consumers can obtain current display names without extra Slack or cachet.dunkirk.sh calls, with a defined sync path for changes.

Written by the indexing model from the issue text.