Device capability and E2EE protocol negotiation

[codebestia]

Write apps/backend/docs/concepts-protocol-negotiation.md covering lib/capabilities.ts and services/e2eeProtocol.ts: how a device advertises supported protocols at registration, how normalizeCapabilities defaults an older client to the sealed-box baseline, how a sender picks a mutually-supported protocol, and how checkEnvelopeProtocols rejects an envelope whose declared protocol the recipient device does not support.

Acceptance criteria:

• Documents the capability payload shape and the BASELINE_PROTOCOL default for clients predating the field
• Explains the per-envelope protocol column and why it is recorded per envelope rather than per device
• Documents the protocol_mismatch rejection and what a client should do on receiving it
• Explains how this enables a staged sealed-box → Signal rollout without breaking older clients

[timilehin01010-byte]

@timilehin01010-byte has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Please assign this issue to me

ℹ️ Repo Maintainers: To accept this application, review their application or assign @timilehin01010-byte to this issue.

[AdaBliss]

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

I’d like to take this issue on. I’ll review the current implementation and relevant project guidelines first, then implement a maintainable solution with tests covering the expected behavior and edge cases

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

[drips-wave]

Congratulations, @AdaBliss! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on August 31, 2026.

🧑‍💻 @AdaBliss: 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 🌊

[github-actions]

Closed by #600, merged into dev.

[drips-wave]

This issue has been completed by @AdaBliss as part of the Stellar Wave Program's 8th Wave 🥳

😎 @AdaBliss: You earned 200 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.