API docs: Conversations endpoints

[codebestia]

Write apps/backend/docs/api-conversations.md covering every route in routes/conversations.ts — list, get single, list messages, create, membership management, mute/archive settings. Document per-device envelope filtering behavior on the message-list routes and cursor-pagination semantics.

Acceptance criteria:

• Every route documented with request/response shape and status codes
• Pagination cursor format and ordering guarantees are explicitly documented
• Per-device ciphertext scoping (why two callers get different messages[] content for the same conversation) is called out explicitly

[Jenks00]

@Jenks00 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hi maintainer I would like to work on this issue please assign it to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Jenks00 to this issue.

[drips-wave]

Congratulations, @Jenks00! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Jenks00: 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.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊