Backend/requests: Support .ics attachments

[tristanlabelle]

Generates and attaches iCalendar .ics files to host request accepted+confirmed emails, which are detected by mail clients like Gmail or manually importable in calendar apps.

Caveats:

• The host receives the .ics file in the host request accepted notification email, even though they haven't confirmed yet, because they don't receive a further email upon confirming that could carry the .ics.
• The surfer receives the .ics file upon the host confirming, which makes sense.
• Upon cancellation, only the party getting cancelled on receives an email with the cancellation .ics, since the cancelling party does not get an email.

Unfortunately the ics library pulls in the arrow and tatsu libraries. Its beefier alternative icalendar depends on python-dateutil and tzdata, which we are already pulling in. I can port over if this is a concern. The format is simple enough for our needs that we could even implement it ourselves.

Fixes #8461

Testing

Run the backend and frontend, and follow these steps:

1. Log in as user A, send a host request to user B, log out
2. Log in as user B, accept the host request, notice in maildev the acceptance email to user A, download the email .eml file, then https://www.encryptomatic.com/viewer/ to extract the attachment, then import it into Gmail from settings, log out
3. Log in as user A, confirm the host request, notice in maildev the confirmation email to user B, validate ics attachment
4. Still as user A, cancel the host request, notice in maildev the confirmation email to user A, validate ics attachment

Also added tests

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 12, 2026 0:46am

[aapeliv]

i love this idea, let me know when it's ready for review

[aapeliv]

i think it looks good overall

[tristanlabelle]

@aapeliv Addressed comments. Ready for ✅ !

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Added calendar invite attachments to host request emails so accepted, confirmed, and cancelled stays can be easily added to calendar apps

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This adds a noticeable user-facing improvement: host request acceptance, confirmation, and cancellation emails can now include calendar invite attachments that users can add to their calendar apps. It directly improves trip planning and is something end users will see and use.