Exchange Integration: PredictIt

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.