Implement T-Bank place_order and get_portfolio (Sandbox)

[Muratich]

Body: Implement broker operations required for Trading Bot on T-Bank sandbox. Historical get_candles stays as-is.

Scope:

• place_order(instrument, action, quantity, price=None) — sandbox implementation
• get_portfolio(account_id=None) — sandbox implementation
• Map responses to existing OrderResult, Position, broker Portfolio models
• Error handling: auth, invalid instrument, rate limit (clear BrokerError)
• No second broker, no adapter factory (Refactor Adapter for Multi-Broker Support #22/Add Second Adapter Implementation #23 deferred)

Definition of Done:

• Integration test or documented manual sandbox script for buy/sell round-trip
• docs/broker_adapter_description.md updated
• Trading Bot can call broker without NotImplementedError