Add Notification Expiration Logic

[Abd-Standard]

Description
Implement expiration support to prevent outdated notifications from being processed or delivered.

Acceptance Criteria
Notifications store an expiration timestamp.
Expired notifications cannot be processed.
Expiration checks are covered by unit tests.

[khanavi272-spec]

@khanavi272-spec has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

I would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @khanavi272-spec to this issue.

[Muneerat]

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

Hi
Please assign me this task.

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

[vicajohn]

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

Hi maintainer, I would love to work on this

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

[drips-wave]

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

🧑‍💻 @vicajohn: 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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