[Epic 7.3] Implement Alpaca trading adapter

[ooples]

User Story

As a trader, I want to execute trades through Alpaca so that I can automate my trading strategy.

Acceptance Criteria

• AlpacaTradeAdapter implements IAutoTradeAdapter
• Configuration via AlpacaOptions:
  • ApiKey (default from env: ALPACA_API_KEY)
  • ApiSecret (default from env: ALPACA_API_SECRET)
  • UsePaper (default: true - paper trading)
  • BaseUrl (auto-selected based on UsePaper)
• Market, Limit, Stop, and Stop-Limit orders
• Position tracking via Alpaca API
• Order status webhooks (optional)
• Fractional shares support

Performance Metrics

Metric	Target
Order submission	<500ms
Position query	<200ms

Dependencies

• [Epic 7.1] Define IAutoTradeAdapter interface

NuGet Dependencies

• Alpaca.Markets (optional)

Safety Notes

• Paper trading is DEFAULT
• Live trading requires explicit UsePaper = false
• Additional confirmation for live trading