Add game performance leaderboard with fair scoring system

[Copilot]

This PR implements a new game leaderboard feature that combines aspects of the existing token leaderboard and individual statistics commands to show the best performing players across casino games.

New Command

/casino tokens game-leaderboard [game]

• Without game parameter: Shows global leaderboard across all games
• With game parameter: Shows leaderboard for specific game (Blackjack, Poker, Rock Paper Scissors)

Key Features

Fair Scoring Formula

Uses winrate × log₁₀(totalGames + 1) to rank players, which solves the core issue where players with just 1 game and 100% winrate would dominate leaderboards. The logarithmic scaling rewards both skill and game volume:

• 1 game, 100% winrate → Score: 30.10 (prevents domination)
• 100 games, 55% winrate → Score: 110.24 (rewards consistency + volume)
• 5 games, 100% winrate → Score: 77.82 (balanced recognition)

Rich Display

Shows top 10 players with:

• 🥇🥈🥉 Medal rankings
• Performance score and game statistics
• Win/loss records and winrates
• Net profit/loss with appropriate icons (💰/💸)

Example Output

🏆 Blackjack Leaderboard
Ranked by performance score (Win Rate × log₁₀(Total Games + 1))

🥇 PlayerName
Score: 110.24
Games: 100 | W/L: 55/45
Win Rate: 55.0% | 💰 Profit: +1,250

Implementation Details

• New service method: CasinoService.GetGameLeaderboard() aggregates user statistics across all users
• New domain model: GameLeaderboardEntry stores calculated scores and performance metrics
• Game name mapping: Properly maps enum values to database-stored game names
• Consistent patterns: Follows existing casino command structure, error handling, and channel permissions

Testing

• ✅ Project builds successfully without errors
• ✅ Scoring formula tested with various scenarios
• ✅ Game name mapping verified against actual database values
• ✅ No breaking changes to existing functionality

Fixes #350.

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[Copilot]

Pull Request Overview

This PR implements a new game performance leaderboard feature that provides a fair ranking system for casino players by combining win rate with game volume using a logarithmic scoring formula. The new /casino tokens game-leaderboard command shows top performers either globally or for specific games.

• Adds GetGameLeaderboard() service method with fair scoring algorithm (winrate × log₁₀(totalGames + 1))
• Implements new /casino leaderboard slash command with optional game filtering
• Creates GameLeaderboardEntry domain model for leaderboard data structure

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
CasinoService.cs	Adds new GetGameLeaderboard() method and updates method signatures to use nullable parameters
CasinoSlashModule.cs	Removes old statistics command and helper method (moved to Games module)
CasinoSlashModule.Games.cs	Adds new leaderboard command, moves statistics command from main module, and adds helper methods
CasinoUser.cs	Adds GameLeaderboardEntry model and updates existing models with nullable reference types