Backend/emails: Support side-by-side buttons in generic emails

[tristanlabelle]

The host request received emails require showing two buttons side by side (view and quick decline). In the generic email system, if we see two "action" blocks in a row, we collapse them into a single "two buttons" block.

1. Modify blocks.mjml to add a two-buttons block, regenerate its html.
2. Update the renderer to merge adjacent ActionBlocks into TwoButtonHTMLBlocks (only for HTML emails).
3. Add slots=True and use asdict

Fixes #8713

Testing

Locally modified an email to add a second action:

        builder.action(donate_link, "donate_action")
        builder.action("https://example.com", "donate_action")

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 20, 2026 8:22am

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This is a backend email template improvement to support side-by-side buttons in emails, mainly enabling a better layout for host request emails. While end users may see a small visual improvement, it is a narrow implementation detail rather than a significant new feature, major UX improvement, or important bug fix affecting many users.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This is a backend email template improvement to support side-by-side buttons in emails, mainly enabling a better layout for host request emails. While end users may see a small visual improvement, it is a narrow implementation detail rather than a significant new feature, major UX improvement, or important bug fix affecting many users.