Backend/emails: Link to terms of service

[tristanlabelle]

I'll follow-up separately for the community guideline since the URL is less obvious.

It's backwards compatible to add a string placeholder: existing translated strings will simply not make use of it.

Testing

Check generated emails linked in this PR. https://02a9ef80--test-artifacts.preview.couchershq.org/emails/index.html

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

[CouchersBot]

Web (Vercel)

View the Vercel web preview for this branch.

Backend

Schema	Schema diff	Sample emails

Other

Protos	Backend coverage

[github-actions]

📝 Release Notes

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

Reason: This change only adds a Terms of Service link to an API key email warning message. It is a minor copy/email improvement affecting a narrow technical audience rather than a significant end-user feature, UX improvement, important bug fix, or major technical work.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This change only adds a Terms of Service link to an API key email warning message. It is a minor copy/email improvement affecting a narrow technical audience rather than a significant end-user feature, UX improvement, important bug fix, or major technical work.