feat: 健康检查加币安 API/WS，订单簿 bids 空防护，自动价差历史取 20 根

- ApiHealthCheckService: 新增币安 API(ping)、币安 WebSocket(5m/15m 连接状态)
- BinanceKlineService: 连接状态追踪 getConnectionStatuses 供健康检查
- CryptoTailOrderbookWsService: book 事件 bids 为空时不再取 [0]，避免 Index 0 out of bounds
- BinanceKlineAutoSpreadService + 文档 + i18n: 历史 K 线由 30 根改为 20 根

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

frontend/src/locales/en/common.json

@@ -1447,7 +1447,7 @@
       "timeWindowStartLEEnd": "Window start must not be greater than end",
       "timeWindowExceed": "Time window must not exceed period length",
       "minSpreadMode": "Min spread",
-      "minSpreadModeTip": "Whether to place an order is based on the spread between open and close in the current period. Auto: system computes a suggested spread from the last 30 klines (updated each period); Fixed: you enter a value (e.g. 30), order only when spread ≥ that value; None: no spread check, order when price is in range.",
+      "minSpreadModeTip": "Whether to place an order is based on the spread between open and close in the current period. Auto: system computes a suggested spread from the last 20 klines (updated each period); Fixed: you enter a value (e.g. 30), order only when spread ≥ that value; None: no spread check, order when price is in range.",
       "minSpreadModeNone": "None",
       "minSpreadModeFixed": "Fixed",
       "minSpreadModeAuto": "Auto",

frontend/src/locales/zh-CN/common.json

@@ -1446,7 +1446,7 @@
       "timeWindowStartLEEnd": "时间区间开始不能大于结束",
       "timeWindowExceed": "时间区间不能超过周期长度",
       "minSpreadMode": "最小价差",
-      "minSpreadModeTip": "根据当前周期开盘价与收盘价的价差决定是否下单。自动：系统按历史 30 根 K 线计算建议价差（每周期更新）；固定：您输入一个数值（如 30），仅当价差 ≥ 该值时才下单；无：不校验价差，满足价格区间即下单。",
+      "minSpreadModeTip": "根据当前周期开盘价与收盘价的价差决定是否下单。自动：系统按历史 20 根 K 线计算建议价差（每周期更新）；固定：您输入一个数值（如 30），仅当价差 ≥ 该值时才下单；无：不校验价差，满足价格区间即下单。",
       "minSpreadModeNone": "无",
       "minSpreadModeFixed": "固定",
       "minSpreadModeAuto": "自动",

frontend/src/locales/zh-TW/common.json

@@ -1447,7 +1447,7 @@
       "timeWindowStartLEEnd": "時間區間開始不能大於結束",
       "timeWindowExceed": "時間區間不能超過週期長度",
       "minSpreadMode": "最小價差",
-      "minSpreadModeTip": "依當前週期開盤價與收盤價的價差決定是否下單。自動：系統依歷史 30 根 K 線計算建議價差（每週期更新）；固定：您輸入一個數值（如 30），僅當價差 ≥ 該值時才下單；無：不校驗價差，滿足價格區間即下單。",
+      "minSpreadModeTip": "依當前週期開盤價與收盤價的價差決定是否下單。自動：系統依歷史 20 根 K 線計算建議價差（每週期更新）；固定：您輸入一個數值（如 30），僅當價差 ≥ 該值時才下單；無：不校驗價差，滿足價格區間即下單。",
       "minSpreadModeNone": "無",
       "minSpreadModeFixed": "固定",
       "minSpreadModeAuto": "自動",