Split exchange and strategy modules for future Binance support.

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

backend/app/exchange/binance/__init__.py

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+from .adapter import BinanceExchange
+
+__all__ = ["BinanceExchange"]

backend/app/exchange/binance/adapter.py

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+"""币安交易所适配器占位：后期接入，接口与 OKX 对齐。"""
+
+from __future__ import annotations
+
+from typing import Any, Sequence
+
+from ...config import Settings, get_settings
+from ..types import BookLevel, MarketSnapshot, OptionPair, Quote
+
+
+class BinanceExchange:
+    name = "binance"
+
+    def __init__(self, settings: Settings | None = None) -> None:
+        self.settings = settings or get_settings()
+
+    async def start(self) -> None:
+        raise NotImplementedError("币安交易所模块尚未接入，请配置 EXCHANGE=okx")
+
+    async def stop(self) -> None:
+        return
+
+    def list_option_contracts(self, family: str) -> list[dict[str, Any]]:
+        raise NotImplementedError("BinanceExchange.list_option_contracts")
+
+    def fetch_index(self, index_id: str) -> float | None:
+        raise NotImplementedError("BinanceExchange.fetch_index")
+
+    def fetch_mark(self, inst_id: str) -> float | None:
+        raise NotImplementedError("BinanceExchange.fetch_mark")
+
+    def fetch_book(
+        self, inst_id: str, depth: int = 5
+    ) -> tuple[list[BookLevel], list[BookLevel], int | None]:
+        raise NotImplementedError("BinanceExchange.fetch_book")
+
+    def get_ct_mult(self, option_inst_id: str, family: str, default: float) -> float:
+        return float(default)
+
+    def set_pair(self, pair: OptionPair | None) -> None:
+        raise NotImplementedError("BinanceExchange.set_pair")
+
+    def warm_and_subscribe(self, inst_ids: Sequence[str]) -> None:
+        raise NotImplementedError("BinanceExchange.warm_and_subscribe")
+
+    async def resubscribe(self, inst_ids: Sequence[str]) -> None:
+        raise NotImplementedError("BinanceExchange.resubscribe")
+
+    def quote(self, inst_id: str) -> Quote | None:
+        return None
+
+    def snapshot(self, perp_inst_id: str) -> MarketSnapshot:
+        raise NotImplementedError("BinanceExchange.snapshot")
+
+    def snapshot_dict(self, perp_inst_id: str) -> dict[str, Any]:
+        raise NotImplementedError("BinanceExchange.snapshot_dict")
+
+    def set_index_px(self, px: float | None) -> None:
+        return
+
+    def set_mark_px(self, inst_id: str, mark_px: float | None) -> None:
+        return