Concepts docs: Presence, heartbeat & connection liveness

[codebestia]

Write apps/backend/docs/concepts-presence-heartbeat.md explaining services/presence.ts and services/heartbeat.ts — the Redis-backed per-device/per-user presence model, TTL-based liveness, the 30s/90s heartbeat timing, and how multi-device aggregation works.

Acceptance criteria:

• Explains the Redis key structure used for presence tracking (which keys exist, what each holds, TTLs)
• Explains the heartbeat timing contract with the actual current interval/timeout values
• Explains multi-device aggregation and the transition-broadcast behavior

[Samuel-21-eng]

@Samuel-21-eng has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Can I work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Samuel-21-eng to this issue.

[drips-wave]

Congratulations, @Samuel-21-eng! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

🧑‍💻 @Samuel-21-eng: 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 🌊