Split exchange and strategy modules for future Binance support.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/exchange/__init__.py

@@ -0,0 +1,16 @@
+"""交易所模块：OKX 已接入，币安占位。策略不直接依赖具体交易所。"""
+
+from .factory import build_exchange, get_exchange, set_exchange
+from .protocol import ExchangeMarket
+from .types import BookLevel, MarketSnapshot, OptionPair, Quote
+
+__all__ = [
+    "BookLevel",
+    "ExchangeMarket",
+    "MarketSnapshot",
+    "OptionPair",
+    "Quote",
+    "build_exchange",
+    "get_exchange",
+    "set_exchange",
+]