Add endpoint with similar markets

We have implementation for fiding similar markets (based on question) in one of the agents here: https://github.com/gnosis/prediction-market-agent/blob/main/prediction_market_agent/db/pinecone_handler.py#L150.

Complication is that this is in the PMA repository, which isn't installable through pypi, but maybe you can install it through git directly into labs-api, not a big trouble for me, we are already doing [that with predicitonprophet for example](https://github.com/gnosis/prediction-market-agent/blob/e5f2f80b98fc39c9c2f8a0025466e72e68d3131a/pyproject.toml#L53).

So, the task here is to use that in a new endpoint.

Start by reading the similar-market implementation in prediction_market_agent/db/pinecone_handler.py around line 150, then inspect labs-api's dependency configuration and the predictionprophet Git dependency example in the linked pyproject.toml. Add a new API endpoint that exposes similar markets based on a question, with the endpoint behavior and dependency integration confirmed by the project tests or API checks.

Written by the indexing model from the issue text.