Transactional email service integration + templates

[joelpeace48-cell]

Epic: Notifications & Messaging

Summary

Integrate an email provider with versioned templates for claims, unlocks, and operator digests.

Background & Problem

Email is a core notification channel; templated + monitored delivery is table stakes.

Acceptance Criteria

• Provider integrated with retry + bounce handling.
• Versioned templates.
• Deliverability monitored.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.