⚡ Performance improvements for /api/Leaderboard/Get

[jernejk]

1. What triggered this change? (PBI link, Email Subject, conversation + reason, etc)

✏️ #1119
This endpoint is called very frequently and can be very slow.

2. What was changed?

✏️ Rewrite query to be about 2x faster on my local machine.

When using 10 threads for 60 seconds it went from 6.8 req/sec to 15.5 req/sec. That's about 2.8 times faster under load.

Based on SQL Server execution plan analysis.

Query	Memory grant	Memory Used	Estimated CPU Cost	CPU time
Old	283 MB	8.5MB	0.954	108 ms
New	9.2 MB	9 MB	0.15	48 ms

Also, fixed some incorrect calculations like balance.

Benchmark test:

bombardier -c 10 -d 60s -m GET `
-H "Authorization: Bearer {{ TOKEN }}" `
https://localhost:5001/api/Leaderboard/Get

3. Did you do pair or mob programming?

✏️ No.

[vladislav-kir]

Looks good to me