API docs: Devices & prekeys endpoints

[codebestia]

Write apps/backend/docs/api-devices.md covering every route in routes/devices.ts (list devices, register, revoke, logout-everywhere, prekey upload) and the legacy-alias routes in routes/userDevices.ts. Include the prekey batch-upload contract (signed vs one-time prekeys, the 200-OTP cap and trimming behavior) and the side effects of revocation (prekey deletion, device_revoked:* publish, key-change system event).

Acceptance criteria:

• Every route's request/response shape, status codes, and ownership/authorization checks are documented
• Prekey upload documents the signed-vs-one-time distinction and cap/trim behavior precisely
• Revocation side effects are documented as part of the DELETE /:id and logout-everywhere entries
• Verified against the current routes/devices.ts and routes/userDevices.ts

[JesseJohn7]

@JesseJohn7 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Please assign me to this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @JesseJohn7 to this issue.

[Lazaruster]

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

I will love to work on this issue

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

[drips-wave]

Congratulations, @JesseJohn7! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @JesseJohn7: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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