Backend/requests: Fix malformed ics attachment

[tristanlabelle]

Fixes a bug introduced in #8635 whereas cancellation ics files would contain both SEQUENCE:0 and SEQUENCE:1, thus interfering with correct cancellation (Gmail wouldn't have it).

Testing

Added more testing, but full E2E testing with Gmail requires me merging and using NEXT.

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

Web frontend checklist

• There are no console warnings when running the app
• Added tests where relevant
• Clicked around my changes running locally and it works
• Checked Desktop, Mobile and Tablet screen sizes

For maintainers

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

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Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 17, 2026 1:25am

[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 user-facing bug affecting host request calendar cancellations in Gmail/Google Calendar, which is noticeable and important for people relying on calendar email attachments.