Backend/requests: Fix calendar event cancellation

[tristanlabelle]

See #8557, host request cancellation was detected by Gmail but failed to reflect in Gcal. Gemini helped me figure it out.

Fixes #8557

Testing

Manually tweaked events and imported them successively into Gcal to observe correct removal. Will also test post-merge (MailDev doesn't have the Gcal tie-in).

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 16, 2026 2:45pm

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Fixed a bug that prevented cancelled host requests from being properly removed from Google Calendar

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This fixes a real user-facing bug where cancelled host request calendar events were not properly removed from Google Calendar, affecting people who rely on calendar invites from request emails.