[Backend] Add doc comments to model structs

[Yunusabdul38]

Description: Improve the understandability of the database layer by adding doc comments to the Rust model structs representing our schema. Tasks:

Add /// comments to all structs in server/src/models/.
Document the primary purpose of each model and its key fields.
Explain status enums or relationships (e.g., Event to Organizer).

[Rajkoli145]

@Rajkoli145 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hi @Agora-Events,
I’d like to work on this issue.

I’ll add clear and consistent doc comments to the model structs to improve readability and maintainability, keeping them aligned with the existing code style.

Happy to follow any specific documentation conventions if preferred.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Rajkoli145 to this issue.

[mikevill20]

@mikevill20 has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

Hello Maintainer, please assign this issue to me. Thanks

ℹ️ Repo Maintainers: To accept this application, review their application or assign @mikevill20 to this issue.

[Simongodw]

@Simongodw has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

I’ll ensure everything aligns with existing conventions and is useful for both developers and documentation tooling.

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

[Chibey-max]

@Chibey-max has applied to work on this issue as part of the Stellar Wave Program's 3rd wave.

i would like to work on this

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

[drips-wave]

Congratulations, @Chibey-max! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on March 30, 2026.

🧑‍💻 @Chibey-max: 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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