Add access control for leaderboard queries

[Samuel1-ona]

Optionally restrict leaderboard visibility to registered players only.

Acceptance Criteria:

• Add leaderboard_visibility config to Hunt
• Options: Public, RegisteredOnly, CreatorOnly
• Enforce visibility in get_hunt_leaderboard

[drips-wave]

Congratulations, @jessiE-bliz445! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @jessiE-bliz445: 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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