feat: add DeepSeek and Xiaomi MiMo LLM provider presets

- Add providers.py with 5 provider presets (OpenAI, DeepSeek, Xiaomi MiMo, Alibaba DashScope, MiniMax)
- Add LLM_PROVIDER env var for one-line provider switching
- Improve <think> tag stripping for reasoning models
- Add .env.example with documented configuration
- Update README with provider configuration section

backend/app/config.py

@@ -17,6 +17,47 @@ else:
     load_dotenv(override=True)
 
 
+def _resolve_llm_config() -> tuple[str, str, str | None]:
+    """
+    解析LLM配置。
+
+    优先级：
+      1. 如果设置了 LLM_PROVIDER，使用提供商预设填充 base_url 和 model（但可被显式值覆盖）
+      2. 否则使用 LLM_BASE_URL / LLM_MODEL_NAME（兼容原有行为）
+
+    Returns:
+        (base_url, model_name, provider_name_or_none)
+    """
+    from .providers import get_provider
+
+    provider_name = os.environ.get('LLM_PROVIDER', '').strip()
+    explicit_base_url = os.environ.get('LLM_BASE_URL', '').strip()
+    explicit_model = os.environ.get('LLM_MODEL_NAME', '').strip()
+
+    if provider_name:
+        preset = get_provider(provider_name)
+        if preset is None:
+            provider_names = ["openai", "deepseek", "xiaomi_mimo", "alibaba_dashscope", "minimax"]
+            available = ", ".join(provider_names)
+            raise ValueError(
+                f"未知的 LLM_PROVIDER: '{provider_name}'. "
+                f"可用值: {available}"
+            )
+        # 显式值优先于预设默认值
+        base_url = explicit_base_url or preset.base_url
+        model = explicit_model or preset.default_model
+        return base_url, model, provider_name
+    else:
+        # 兼容原有行为：无 LLM_PROVIDER 时直接使用显式值
+        base_url = explicit_base_url or 'https://api.openai.com/v1'
+        model = explicit_model or 'gpt-4o-mini'
+        return base_url, model, None
+
+
+# 在模块加载时解析配置（避免重复计算）
+_llm_base_url, _llm_model_name, _llm_provider = _resolve_llm_config()
+
+
 class Config:
     """Flask配置类"""
     
@@ -29,8 +70,9 @@ class Config:
     
     # LLM配置（统一使用OpenAI格式）
     LLM_API_KEY = os.environ.get('LLM_API_KEY')
-    LLM_BASE_URL = os.environ.get('LLM_BASE_URL', 'https://api.openai.com/v1')
-    LLM_MODEL_NAME = os.environ.get('LLM_MODEL_NAME', 'gpt-4o-mini')
+    LLM_PROVIDER = _llm_provider          # e.g. "deepseek" or None
+    LLM_BASE_URL = _llm_base_url
+    LLM_MODEL_NAME = _llm_model_name
     
     # Zep配置
     ZEP_API_KEY = os.environ.get('ZEP_API_KEY')
@@ -73,3 +115,19 @@ class Config:
             errors.append("ZEP_API_KEY 未配置")
         return errors
 
+    @classmethod
+    def get_active_provider_info(cls) -> dict:
+        """返回当前活跃的LLM提供商信息（用于日志/API展示）"""
+        from .providers import get_provider
+
+        info = {
+            "provider": cls.LLM_PROVIDER or "custom",
+            "base_url": cls.LLM_BASE_URL,
+            "model": cls.LLM_MODEL_NAME,
+        }
+        if cls.LLM_PROVIDER:
+            preset = get_provider(cls.LLM_PROVIDER)
+            if preset:
+                info["display_name"] = preset.display_name
+                info["notes"] = preset.notes
+        return info