implement xlm/usd price conversion service

[A6dulmalik]

• Created src/stellar/price.service.ts with getXLMUSDRate() and convertUSDToXLM() methods
• Created src/stellar/stellar.module.ts to export PriceService
• Added totalAmountXLM column to TicketPurchase entity
• Integrated PriceService into TicketPurchaseService to populate XLM amount on order creation
• Added environment configuration for price feed source (CoinGecko or custom API)
• Implemented 60-second caching using @nestjs/cache-manager
• Added comprehensive unit tests covering cache hit/miss, API integration, and error handling

closes #427