Add `device_prekeys` schema (signed prekey + one-time prekeys)

[codebestia]

Create device_prekeys table: id uuid PK, deviceId uuid FK → user_devices.id (cascade), keyType pgEnum (signed/one_time), keyId integer, publicKey text, signature text nullable, consumed boolean default false, createdAt. Indexes: unique (deviceId, keyType, keyId); partial on (deviceId) WHERE key_type='one_time' AND consumed=false. Signed prekeys require non-null signature.

Acceptance criteria:

• One device has exactly one active signed prekey and many one-time prekeys
• Consuming a one-time prekey flips consumed=true atomically
• Signed prekey rows require a non-null signature

[N-thnI]

@N-thnI has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Dear maintainer
Please assign me this issue

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

[Ceejaytech25]

@Ceejaytech25 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

GM GM Maintainer, pls can I work on this issue ? ETA -24hrs

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Ceejaytech25 to this issue.

[drips-wave]

Congratulations, @Ceejaytech25! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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