Implement in-game leaderboards.

[RealistikDash]

Geometry Dash features opt-in leaderboards available on a per-level basis.

These allow the user to view and submit their current progress on a level through an endpoint called by the client. These are sorted by:

• The percentage completion
• The number of coins

This also allows for a potential server-side anticheat, as these leaderboards often see basic level abuse:

• Verifying coin counts match the level's max.
• Verifying coins can only be obtained at 100%

Potential things to look into:

• Might be a bit overkill but using Redis to store the order for quick lookups.
• Wipe leaderboards on level update. I don't think the official game does this but it may be a good idea.