Add celery config to adjust retry settings

[audiodude]

I've gotten multiple user reports now that previews have "hung" and not completed. This is definitely an issue with the Celery worker.

Looking in the worker logs, I see:

redis.exceptions.ConnectionError: Error while reading from fly-rainfall-redis.upstash.io:6379 : (104, 'Connection reset by peer')

It seems like it's losing the connection to the Upstash Redis. In this PR, I've attempted to stop using a worker pool (so connections don't get stale), use infinite retries, and retry on startup.

Hopefully this will fix the issue.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 96.59%. Comparing base (90fade6) to head (6f5da35).

Files with missing lines	Patch %	Lines
rainfall/celeryconfig.py	0.00%	6 Missing ⚠️

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
==========================================
- Coverage   96.78%   96.59%   -0.20%     
==========================================
  Files          37       38       +1     
  Lines        2958     2965       +7     
==========================================
+ Hits         2863     2864       +1     
- Misses         95      101       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.