Export Price Series from Data Loader for Composable Engine

[Muratich]

Depends #116

Data loader can keep full OHLCV in SQLite. Composable engine needs (timestamp, price)[] at runtime.

Scope:

• Add load_price_series(instrument, timeframe, from_dt, to_dt) -> list[PriceBar] (or return (timestamps[], prices[]))
• Reuse existing DB query + cache; extract close + normalized timestamp
• Wire into main.py path used before ExecutionEngine.run() / GridOptimizer (Muratich integrates call site)

Definition of Done:

• Single function callable from main.py without duplicating SQL in strategy module
• Unit test: DB fixture → correct price series length and values
• prepare_candles() / validator still run on full candles before storage
• README updated in src/data_loader/