Exchange Integration: PredictIt

[realfishsam]

Implement support for PredictIt.

PredictIt is mentioned in the README with its logo but currently lacks an implementation class in src/exchanges.

Tasks:

• Create PredictItExchange class extending PredictionMarketExchange.
• Implement fetchMarkets using PredictIt's public API.
• Implement searchMarkets.
• Map outcomes and data to UnifiedMarket format.