fix(billing): handle string usage limits TASK-1232

[jamesrkiger]

🗒️ Checklist

1. run linter locally
2. update all related docs (API, README, inline, etc.), if any
3. draft PR with a title <type>(<scope>)<!>: <title> TASK-1234
4. tag PR: at least frontend or backend unless it's global
5. fill in the template below and delete template comments
6. review thyself: read the diff and repro the preview as written
7. open PR & confirm that CI passes
8. request reviewers, if needed
9. delete this section before merging

📣 Summary

Fixes backend code to handle usage limits that come from Stripe metadata as strings representing either numbers or "unlimited" usage.

👀 Preview steps

1. Create a user and sign them up for a paid subscription
2. Make a project with an audio question and make a submission with an audio file
3. Navigate to NLP processing for that answer. Manually write a short transcription.
4. On billing-addons-backend branch, try running an automatic translation of that transcription. You will get a 500 error from the backend because the string for the usage limit is being compared to an int.
5. On this branch, you will get a successful transcription.

💭 Notes

I would like to have set up more unit testing related to 'unlimited' usage limits, but the only two usage types that can have unlimited usage currently (and there is no plan to change this) are submissions and storage. We do not currently check plan limits for these usage types on the backend, so there are no code paths for testing the 'unlimited' change in this PR.

[notion-workspace]

Handle string values/potentially ‘unlimited’ usage limits