User notification preferences + unsubscribe compliance

[joelpeace48-cell]

Epic: Notifications & Messaging

Summary

Let users control channels/frequency and honor unsubscribe requirements.

Background & Problem

Respecting preferences is required for compliance and trust.

Acceptance Criteria

• Preferences persist + are honored.
• Unsubscribe works across channels.

---

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