Track `lastSeenAt` per device on activity

[codebestia]

Update user_devices.lastSeenAt on socket connect/heartbeat, throttled to at most once per 60s per device to avoid write amplification.

Acceptance criteria:

• lastSeenAt advances on activity but writes are throttled
• No write storm under frequent heartbeats

[ndyugwu]

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

i would like to work on this kindly assign me

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

[Israel-light]

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

I'd like to work on this issue.

It has a clear scope and addresses an important performance concern by keeping device activity information up to date while avoiding excessive database writes from frequent heartbeat events.

I have experience working with backend application logic and I'm interested in implementing a clean throttling approach that updates lastSeenAt only when necessary. I'll make sure the solution handles both socket connections and heartbeat activity while preventing write amplification under high-frequency events.

I'll also add tests to verify that activity updates are recorded correctly and that repeated heartbeats do not generate unnecessary writes.

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

[Agbeleshe]

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

Hey, can I solve this? My plan is to explore the current implementation, identify the most suitable integration points, and develop a clean, efficient solution. I'll make sure the feature is properly tested and aligns with the project's coding standards.

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

[drips-wave]

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

🧑‍💻 @Agbeleshe: 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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