[P1][backend] Add timezone-safe daily reward logic

[dzekojohn4]

Use a defined user/server timezone policy, avoid DST bugs, and test midnight boundaries and repeated claims.

[Maryam-matus]

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

I would love to work on this issue

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

[nottherealalanturing]

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

Hi, I'd like to work on this issue.

I'll implement this following the existing conventions in the codebase, with tests covering the new behavior.

Please assign this issue to me, I'd be happy to contribute.

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

[lovesmilesmall-hue]

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

I'd like to implement timezone-safe daily reward logic. The core deliverable is using a defined user or server timezone policy, avoiding DST bugs, and testing midnight boundaries and repeated claims. My approach is to centralize timezone handling for reward resets and add boundary and idempotency tests.

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

[drips-wave]

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

🧑‍💻 @lovesmilesmall-hue: 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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