`mls_key_packages` schema + upload/fetch endpoints

[codebestia]

Create mls_key_packages (id, deviceId → user_devices.id, package bytea/text, expiresAt, consumed, createdAt). Each device publishes MLS key packages (public). Endpoints: POST /devices/:id/mls-key-packages (upload batch), GET /users/:id/devices/:deviceId/mls-key-package (fetch + consume one for adding to a group). Validate key material server-side.

Acceptance criteria:

• Key packages stored (public only)
• Fetch consumes one package atomically
• Replenishment signal emitted like prekeys

[Ola-Olamijuwon]

@Ola-Olamijuwon has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi there, I can take care of this ASAP.

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

[Olorunfemi20]

@Olorunfemi20 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello! I'm interested in tackling this issue. I've reviewed the task and I'm confident in my ability to deliver a reliable implementation. I'll keep the changes focused, communicate any important findings during development, and submit a well-tested pull request.

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[ndyugwu]

@ndyugwu has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i would like to work on this issue kindly assign me

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[drips-wave]

Congratulations, @Olorunfemi20! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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