Enhance LLM integration by adding OpenAI client support and enabling dynamic routing between DeepSeek and OpenAI services. Update environment configuration to include LLM_SERVICE_CODE for service selection, and modify API server to accommodate new request parameters for language and model. Implement streaming response improvements for chat interactions, allowing for segmented SSE output. Update documentation and impact analysis to reflect these changes.

backend/llm/router.py

@@ -0,0 +1,52 @@
+"""
+LLM Client 路由器：在 DeepSeek / OpenAI 之间切换。
+
+约定：
+- 调用方只依赖 duck-typing：需要 ``chat`` / ``chat_with_json`` 等方法。
+- 通过环境变量 ``LLM_SERVICE_CODE``（openai|deepseek）决定默认路由；
+  若未设置则按 Key 存在性自动选择（优先 deepseek）。
+"""
+
+from __future__ import annotations
+
+import os
+from typing import Any, Optional
+
+from llm.deepseek_client import DeepSeekClient, DeepSeekConfig
+from llm.openai_client import create_openai_client
+
+
+def resolve_llm_service_code(service_code: Optional[str] = None) -> str:
+    sc = (service_code or os.getenv("LLM_SERVICE_CODE") or "").strip().lower()
+    if sc in ("openai", "deepseek"):
+        return sc
+
+    # 自动选择：优先 DeepSeek（与历史默认一致）
+    if (os.getenv("DEEPSEEK_API_KEY") or "").strip():
+        return "deepseek"
+    if (os.getenv("OPENAI_API_KEY") or "").strip():
+        return "openai"
+    return "deepseek"
+
+
+def create_llm_client(service_code: Optional[str] = None, **kwargs: Any) -> Any:
+    """
+    创建 LLM Client。
+
+    Returns:
+        DeepSeekClient 或 OpenAIClient（同形态接口）。
+    """
+    sc = resolve_llm_service_code(service_code)
+    if sc == "openai":
+        # OpenAI 侧：默认从 OPENAI_* 读取
+        return create_openai_client(**kwargs)
+
+    # DeepSeek 侧：从 DEEPSEEK_* 读取
+    api_key = (kwargs.pop("api_key", None) or os.getenv("DEEPSEEK_API_KEY") or "").strip()
+    if not api_key:
+        raise ValueError("未配置 DEEPSEEK_API_KEY（LLM_SERVICE_CODE=deepseek）")
+    base_url = (kwargs.pop("base_url", None) or os.getenv("DEEPSEEK_BASE_URL") or "https://api.deepseek.com").strip()
+    model = (kwargs.pop("model_name", None) or os.getenv("MODEL_PRIMARY") or "deepseek-chat").strip()
+    cfg = DeepSeekConfig(api_key=api_key, base_url=base_url, model_name=model, **kwargs)
+    return DeepSeekClient(cfg)
+