feat: implement buy shares service with AMM integration

[Sundayabel222]

Close #85

@GoSTEAN Pls review Pr

• Add trading.service.ts with buyShares() method for acquiring shares
• Implement slippage protection with configurable min shares parameter (5% default)
• Create Trade entity records with CONFIRMED status for all buy transactions
• Add Share entity creation and updates with cost basis tracking
• Update market totalVolume on each buy transaction atomically
• Add trading.controller.ts with POST /buy endpoint for HTTP requests
• Implement trading routes for buy/sell/odds endpoints
• Integrate with Stellar AMM contract for price execution
• Use Prisma transactions for data consistency and atomicity
• Track user USDC balance changes and position updates
• Add comprehensive error handling (insufficient balance, closed markets, slippage)
• Add 17 passing integration tests covering all acceptance criteria
• All tests verified: buy flow, validation, database updates, share positions

Acceptance Criteria Met:
Create trading.service.ts with buyShares() method
Include slippage protection (min shares param)
Record Trade entity in database
Update/create Share entity
Update market volume
Create trading routes and controller
Unit + integration tests (17 tests passing)