rack：fan 命令放行保持时间 sec，风扇断电显示「停」

Co-Authored-By: Claude Opus 5.5 <[email protected]>

rack/backend/prrack/protocol.py

@@ -172,7 +172,15 @@ def build_command(req: dict) -> dict:
         duty = req.get("duty")
         if not isinstance(duty, int) or isinstance(duty, bool) or not 0 <= duty <= 1000:
             raise CmdError("风扇下限必须是 0~1000‰（0 = 自动）")
-        return {"cmd": cmd, "duty": duty}
+        out = {"cmd": cmd, "duty": duty}
+        # 固件 ≥0.1.21：`sec` = 这个下限保持多少秒，到期自动回落到自动（没给 = 一直保持）。
+        sec = req.get("sec")
+        if sec is not None:
+            if not isinstance(sec, int) or isinstance(sec, bool) or not 0 <= sec <= 86400:
+                raise CmdError("风扇下限的保持时间必须是 0~86400 秒（0 / 不给 = 一直保持）")
+            if sec:
+                out["sec"] = sec
+        return out
     raise CmdError("不支持的命令：%r" % (cmd,))