Switch celery to thread runners for dev for the Macs

[bjester]

Summary

Team members are having issues running the celery task workers. Investigation found issues with process forking:

On Apple Silicon + macOS 26, billiard's prefork pool is fundamentally unreliable regardless of what's loaded. The grpcio chase was a red herring; the SIGSEGV would likely happen even with a minimal task.

The fix is --pool=solo. Looking at the pnpm run celery script, it already uses -c 1 (single worker), so solo is equivalent in behavior — just without the fork. The simplest thing is to run it directly

References

https://learningequality.slack.com/archives/C0LK8QS9J/p1774881025194369

Reviewer guidance

1. Start up the dev server and celery make devceleryworkers
2. Create a channel
3. Add some content to the channel
4. Publish the channel
5. Verify that the task workers ran the tasks