Build complete Market Data backend

[rotemsi1]

@claude
Read all the documents in the planning directory, then build the complete Market Data backend in the backend directory. This should include:

1. The Massive API interface
2. The unified Market Data interface
3. The Market Data simulator
   Include full unit tests

[rotemsi1]

@ed-donner Please ignore. I was just following along with the video from Week 3 Day 2