[P1][backend] Make streak calculations resilient to retries and clock skew

[dzekojohn4]

Use server dates and unique activity keys; test duplicate activities, missed days, DST, and concurrent writes.

[thelight08]

@thelight08 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Please assign me boss😭😭🤲
You never assigned me for job before

ℹ️ Repo Maintainers: To accept this application, review their application or assign @thelight08 to this issue.

[kingeligma]

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

Hello sire, i am intrseted in this issue and would love to cook with it, Grant my request my lord.

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[anifast-123]

@anifast-123 has applied to work on this issue as part of the Stellar Wave Program's 8th wave.

Hello maintainer, i would like to work on this issue

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[gbengaeben]

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

Hello Maintainer
Can you please kindly assign me this issue to fix

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[drips-wave]

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

🧑‍💻 @gbengaeben: 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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