feat(notifications): implement email/SMS/push delivery channels (only in-app today)

[cypriannwokolo2-creator]

Summary

Spec requires email/sms/push channels, but only in-app notifications are implemented; the email/OTP path is not actually wired (see email integration issue).

Acceptance Criteria

• Channel abstraction (in-app, email, SMS, push)
• User preference gating
• Delivery + retry + audit
• Tests

Complexity

High.

[ridwanraji1290-ai]

@ridwanraji1290-ai has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

I’ve reviewed the issue description and have a good understanding of what is required. I’d be happy to take ownership and contribute a solution.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @ridwanraji1290-ai to this issue.

[danfulani0]

@danfulani0 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hi Maintainer! I'm a senior developer with experience building high-quality, production-ready software. I'd love to take on this issue and deliver a clean, well-tested solution that aligns with the project's standards. Please assign it to me—I'm ready to get started and contribute effectively.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @danfulani0 to this issue.