Add detailed market data backend design document

[RuquiaAlam]

Adds planning/MARKET_DATA_DESIGN.md, a full design reference for the market data subsystem covering the unified API, GBM simulator, and Massive (Polygon.io) REST client.

Contents:

• Unified API: MarketDataSource ABC and the shared PriceCache, with the producer-writes/consumer-reads rule that makes sources interchangeable
• Data model: PriceUpdate, derived change/direction properties, SSE serialization
• Simulator: GBM math and dt derivation, Cholesky-correlated sector moves, random shock events, seed prices and per-ticker parameters
• Massive API: snapshot endpoint, rate-limit-driven poll intervals, asyncio.to_thread offloading, two-level error handling
• SSE endpoint: wire format, version-gated change detection, reconnect and disconnect behavior, EventSource client example
• Integration contracts for the API layer: FastAPI lifespan wiring, watchlist coordination, trade fills and portfolio valuation
• Error handling matrix, testing strategy with example tests, tuning reference, and open design items

Code snippets reflect the implementation in backend/app/market/.