Deduplicate leaderboard users whose GitHub usernames differ only by case #183

What happened

The Yemen GitHub Developer Leaderboard lists the same GitHub account more than once when the username differs only by letter case. For example, watheqalshowaiter is shown at rank 2 and WatheqAlshowaiter is shown at rank 3, with the same displayed name and scores.

What I expected

Each GitHub account should appear once in a leaderboard, regardless of the capitalization used for its username. GitHub treats username casing as case-insensitive.

Steps to reproduce

1. Open https://devimpact.osamamabkhot.dev/leaderboard/yemen.
2. Review ranks 2 and 3.
3. Observe that watheqalshowaiter and WatheqAlshowaiter are separate entries for the same GitHub account.

Additional context

Both entries currently display the same developer name, final score (412), repository score (111), PR score (793), and contribution score (51). This also affects ranking positions and can make the leaderboard appear to contain more unique developers than it does.