Implement casino shop feature for token purchases

[Copilot]

This PR implements a comprehensive casino shop system where users can spend their tokens on various items. The shop provides a clean Discord interface for browsing and purchasing items, with full transaction tracking and integration with the existing casino system.

Key Features

Shop Interface

• New /casino shop slash command displays an ephemeral embed with all available items
• Shows user's current token balance and item descriptions with prices
• Uses dropdown selection for purchasing (better UX than buttons for multiple items)
• Items already purchased are displayed as struck-through with ✅ OWNED status
• Items are automatically removed from the dropdown once purchased

Default Shop Items

• Auto Daily Tokens - 20,000 tokens (placeholder for future auto-daily functionality)
• Badge: Casino Lover - 1,000 tokens
• Badge: High Roller - 10,000 tokens
• Badge: Token Master - 100,000 tokens
• Badge: Millionaire - 1,000,000 tokens

Transaction System

• New TransactionType.Shop for tracking token expenditures
• All purchases are logged in the transaction history with item details
• Shop purchases appear in /casino tokens history as "Bought [Item Name]"
• Proper token balance validation prevents overspending
• Duplicate purchase prevention - users can't buy items they already own

Database Schema

• Two new tables: shop_items and shop_purchases
• Foreign key relationships ensure data integrity
• Automatic table creation for both new installations and existing deployments
• Shop purchases are properly cleared during /casino reset

Error Handling

• Comprehensive validation for insufficient funds, invalid items, and duplicate purchases
• User-friendly error messages with clear explanations
• Full exception logging for debugging
• Graceful fallbacks for edge cases

The implementation follows existing code patterns and maintains backward compatibility while extending the casino system with this new functionality.

Fixes #358.

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[Pierre-Demessence]

/deploy_dev

[github-actions]

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