Add leaderboard prize distribution support

[MaryammAli]

- Description: Add backend logic for leaderboards and prize pools.
- Path: `BackendAcademy/src/rewards/`

[GazzyLee]

hello boss, I can get this done. ETD is in 5 hours tops, please asign

[cybermax4200]

@cybermax4200 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Please assign me I can work on this

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[drips-wave]

Congratulations, @cybermax4200! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

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[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@cybermax4200's PR #205 was approved and merged by @MaryammAli.

🏆 @cybermax4200: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (331 total points). Track your full progress on GrantFox.

👏 Great work, @cybermax4200! Keep contributing to BlockDash-Studios.

[drips-wave]

This issue has been completed by @cybermax4200 as part of the Stellar Wave Program's 6th Wave 🥳

😎 @cybermax4200: You earned 100 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

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