feat: 完成 issue #45 ④ 云端 API（Qwen/DeepSeek）接入与安全网关

core/llm-gateway/backends.py

@@ -15,9 +15,10 @@
 from __future__ import annotations
 
 import json
+import os
 import time
 import urllib.request
-from typing import Optional, Sequence
+from typing import Callable, Optional, Sequence
 
 from .gateway import InferenceBackend
 
@@ -112,3 +113,98 @@ class Local70BBackend(InferenceBackend):
         except Exception as exc:  # noqa: BLE001 - 健康探测失败仅记录
             base["status"] = f"error: {exc}"
         return base
+
+
+class CloudApiBackend(InferenceBackend):
+    """云端 API 推理后端（Qwen / DeepSeek 等 OpenAI 兼容）—— issue #45。
+
+    **安全网关约束（PRD 5.4）**：
+    - 仅接收 **DLP 放行**的脱敏/通用内容（上游 `LLMGateway` 主编排出站检查 +
+      cloud 分支输出 DLP 复查）；
+    - API Key 从**环境变量**读取（`api_key_env`），不硬编码、不落日志；
+    - 可选 `safety_checker` 出站复查钩子（fail-closed：复查拒绝 → 拦截占位，
+      不调用上游）。
+    """
+
+    name = "cloud-api"
+
+    def __init__(
+        self,
+        endpoint: str = "",
+        api_key_env: str = "",
+        model: str = "deepseek-chat",
+        timeout_seconds: float = 60.0,
+        max_tokens: int = 1024,
+        temperature: float = 0.1,
+        safety_checker: Optional[Callable[[str], bool]] = None,
+    ) -> None:
+        self.endpoint = (endpoint or "").rstrip("/")
+        self.api_key_env = api_key_env
+        self.model = model
+        self.timeout = float(timeout_seconds)
+        self.max_tokens = int(max_tokens)
+        self.temperature = float(temperature)
+        # 出站安全复查：返回 False 即拦截（fail-closed）
+        self.safety_checker = safety_checker
+        self._api_key = os.environ.get(api_key_env, "") if api_key_env else ""
+
+    # ------------------------------------------------------------------
+    def generate(self, prompt: str, context: Sequence[str]) -> str:
+        """生成回答。安全网关：safety_checker 拒绝 → 拦截占位，不调用上游。"""
+        if self.safety_checker is not None and not self.safety_checker(prompt):
+            return "[云端安全网关拦截] 出站复查未通过，已拦截（数据不出厂）。"
+
+        if not self.endpoint:
+            return self._dry_run(prompt, context)
+
+        payload = {
+            "model": self.model,
+            "messages": [
+                {"role": "system", "content": self._system_prompt(context)},
+                {"role": "user", "content": prompt},
+            ],
+            "max_tokens": self.max_tokens,
+            "temperature": self.temperature,
+        }
+        body = self._post_json("/v1/chat/completions", payload)
+        try:
+            return body["choices"][0]["message"]["content"]
+        except (KeyError, IndexError, TypeError):
+            raise RuntimeError(
+                f"云端 API 响应格式异常: {str(body)[:200]}")
+
+    # ------------------------------------------------------------------
+    def _system_prompt(self, context: Sequence[str]) -> str:
+        refs = "\n".join(f"- {c}" for c in (context or []))
+        base = "你是工业 AI 助手。回答须基于给定资料并标注来源。"
+        return f"{base}\n参考资料：\n{refs}" if refs else base
+
+    def _dry_run(self, prompt: str, context: Sequence[str]) -> str:
+        head = f"[云端API占位] {prompt[:40]}"
+        for i, src in enumerate(context[:3], 1):
+            head += f"\n[来源: {src}]"
+        if self.safety_checker is not None:
+            head += "\n[安全网关: 已复查放行]"
+        return head
+
+    def _post_json(self, path: str, payload: dict) -> dict:
+        """向后端推理服务发起 JSON POST（Bearer 认证，Key 来自环境变量）。"""
+        url = self.endpoint + path
+        data = json.dumps(payload).encode("utf-8")
+        headers = {"Content-Type": "application/json"}
+        if self._api_key:
+            headers["Authorization"] = f"Bearer {self._api_key}"
+        req = urllib.request.Request(url, data=data, headers=headers)
+        with urllib.request.urlopen(req, timeout=self.timeout) as resp:
+            raw = resp.read().decode("utf-8")
+        return json.loads(raw) if raw else {}
+
+    def health(self) -> dict:
+        """后端健康信息（含安全网关状态，不含密钥）。"""
+        return {
+            "backend": self.name, "model": self.model,
+            "endpoint": self.endpoint or "(dry-run)",
+            "api_key_configured": bool(self._api_key),
+            "safety_checker": self.safety_checker is not None,
+            "status": "dry-run" if not self.endpoint else "configured",
+        }