Fix N+1 in runner jobs API

What does this MR do and why?

Adds preloads to prevent N+1 database and Gitaly calls.

This endpoint was showing up in #mech_symp_alerts. URL to logs here (internal)

We can see from the logs that these slow requests execute ~700 SQL queries and ~40 Gitaly calls

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.