Fix Binance option top-of-book via depth limit and ticker fallback.

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

backend/app/config.py

@@ -37,6 +37,7 @@ class Settings(BaseSettings):
     binance_fapi_base: str = "https://fapi.binance.com"
     binance_eapi_base: str = "https://eapi.binance.com"
     binance_futures_ws: str = "wss://fstream.binance.com/stream"
+    # 官方默认根路径 /eoptions；组合流用 /eoptions/stream?streams=
     binance_options_ws: str = "wss://nbstream.binance.com/eoptions/stream"
     binance_http_proxy: str = ""