Split exchange and strategy modules for future Binance support.

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

backend/app/strategy/engine.py

@@ -8,7 +8,7 @@ import time
 from typing import Any
 
 from ..config import get_settings
-from ..market import get_gateway
+from .session import get_session
 from ..models.db import get_db
 from ..sim.ledger import Ledger
 from ..sim.matcher import Matcher
@@ -131,7 +131,7 @@ class StrategyEngine:
                     continue
                 async with self._lock:
                     try:
-                        await get_gateway().ensure_atm_async(force=False)
+                        await get_session().ensure_atm_async(force=False)
                     except Exception as e:
                         logger.warning("ATM ensure before tick failed: %s", e)
                     await self._tick_async()
@@ -191,8 +191,7 @@ class StrategyEngine:
             self._set_state(phase="idle")
 
         self._set_state(phase="wait_signal")
-        gw = get_gateway()
-        pick = await gw.pick_for_open_async()
+        pick = await get_session().pick_for_open_async()
         if pick is None:
             self._set_state(
                 last_error="无合格期权：需剩余时长与杠杆倍数同时满足"