fix: make email handler async and return response immediately to avoid timeout (#373)

## What does this PR do?

Mailbridge email handler waited syncronously to process emails in the new version, it lead yo timeouts.
Now it returns response immediately and handles the email in backgorund

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## Type of change

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- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] Chore (refactoring code, technical debt, workflow improvements)
- [ ] Enhancement (small improvements)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
- [ ] This change requires a documentation update

## Checklist

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### Required

- [ ] Read [Contributing Guide](https://github.com/un/inbox/blob/main/CONTRIBUTING.md)
- [ ] Self-reviewed my own code
- [ ] Tested my code in a local environment
- [ ] Commented on my code in hard-to-understand areas
- [ ] Checked for warnings, there are none
- [ ] Removed all `console.logs`
- [ ] Merged the latest changes from main onto my branch with `git pull origin main`
- [ ] My changes don't cause any responsiveness issues

### Appreciated

- [ ] If a UI change was made: Added a screen recording or screenshots to this PR
- [ ] Updated the UnInbox Docs if changes were necessary

Author: BlankParticle