Backend/emails: Take links out of loc files.

[tristanlabelle]

Having html links in loc strings introduces risks that translators make mistakes with the syntax, and makes it more difficult to modify the URLs. It's more maintainable to interpolate the links from the code.

Testing

Checked at generated emails on this PR:

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]

Mobile

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Deep links

iOS

couchers-devtool://expo-development-client/?url=https%3A%2F%2Fb3f08c21--ota.preview.couchershq.org%2Fios%2Fmanifest

Android

couchers-devtool://expo-development-client/?url=https%3A%2F%2Fb3f08c21--ota.preview.couchershq.org%2Fandroid%2Fmanifest

Web (Vercel)

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Backend

Schema	Schema diff	Sample emails

Other

Protos	Backend coverage	Web coverage

[aapeliv]

lgtm otherwise

[tristanlabelle]

Fixed email address and rebased. Merging based on "lgtm otherwise"

[github-actions]

📝 Release Notes

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

Reason: This PR is an internal maintainability refactor for email localization and link handling. It does not add new user-facing functionality or fix a noticeable bug for end users, and the impact is mainly reducing translator error risk in the codebase.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This PR is an internal maintainability refactor for email localization and link handling. It does not add new user-facing functionality or fix a noticeable bug for end users, and the impact is mainly reducing translator error risk in the codebase.