Add email template validation on startup

[hman38705]

Issue 043: Add email template validation on startup

Area: Backend
Priority: Medium
Files: services/api/src/email/service.rs, services/api/templates/

Problem

Email templates are loaded lazily and errors surface at send time. Template syntax errors cause runtime failures.

Acceptance Criteria

• Templates are validated at startup\n- Invalid templates prevent server start with clear error\n- Template variables are documented\n- CI validates templates

[Haroldwonder]

@Haroldwonder has applied to work on this issue as part of the Stellar Wave Program's 4th wave.

I can handle this.

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[drips-wave]

Congratulations, @Haroldwonder! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on April 29, 2026.

🧑‍💻 @Haroldwonder: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

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[drips-wave]

This issue has been completed by @Haroldwonder as part of the Stellar Wave Program's 4th Wave 🥳

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