Backend/emails: Support custom SMTP headers for attachments

[tristanlabelle]

ics attachments are trickier than I thought, one of the ics field (method) also needs to be replicated as an SMTP header on the attachment for correct support by email clients. EmailAttachment wasn't flexible enough to represent this, so I switched it over to specifying the Content-Type and Content-Disposition headers directly, which is as generic as it gets. This should improve calendar event compatibility and behavior in email clients.

There's some extra dance to support unquoted method=XXX in the SMTP header (not the default Python behavior) since Gemini said that was preferred for backcompat with old email clients.

Testing

Ran locally, accepted host requests and downloaded emails from MailDev while iterating until I got this desired form for the attachment:

--===============2710363521157735036==
Content-Type: text/calendar; charset="utf-8"; method=PUBLISH
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename="host_request.ics"
MIME-Version: 1.0

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

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Project	Deployment	Actions	Updated (UTC)
couchers	Ready	Preview	May 26, 2026 8:33pm

[tristanlabelle]

Rebased to resolve conflicts (adjacent import lines had changed in render_email.py)

[github-actions]

📝 Release Notes

This PR should be included in the release notes.

Suggested release note:

Improved calendar email attachments so host request events work more reliably across email and calendar apps

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: include
Reasoning: This is an important user-facing bug fix: it improves .ics calendar attachment compatibility in email clients for host request emails, making calendar events behave correctly when users receive them. While technical, the impact is noticeable to end users who add or remove hosting events from their calendars.