配置 API Key 环境变量，完成 FastAPI 基础配置

backend/app/config.py

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+from pydantic_settings import BaseSettings, SettingsConfigDict
+from pathlib import Path
+
+class Settings(BaseSettings):
+    # 应用基础配置
+    APP_NAME: str = "FilesReadSystem"
+    DEBUG: bool = True
+    API_V1_STR: str = "/api/v1"
+    
+    # 数据库
+    MONGODB_URL: str
+    MONGODB_DB_NAME: str
+    REDIS_URL: str
+    
+    # AI 相关
+    LLM_API_KEY: str
+    LLM_BASE_URL: str
+    LLM_MODEL_NAME: str
+    
+    # 文件路径
+    BASE_DIR: Path = Path(__file__).resolve().parent.parent.parent
+    UPLOAD_DIR: str = "data/uploads"
+    
+    # 允许 Pydantic 从 .env 文件读取
+    model_config = SettingsConfigDict(
+        env_file=Path(__file__).parent.parent / ".env", 
+        env_file_encoding='utf-8',
+        extra='ignore'
+    )
+
+settings = Settings()

backend/app/main.py

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+from fastapi import FastAPI
+from config import settings
+
+app = FastAPI(
+    title=settings.APP_NAME,
+    openapi_url=f"{settings.API_V1_STR}/openapi.json"
+)
+
+@app.get("/")
+async def root():
+    return {
+        "message": f"Welcome to {settings.APP_NAME}",
+        "status": "online",
+        "debug_mode": settings.DEBUG
+    }
+
+if __name__ == "__main__":
+    import uvicorn
+    uvicorn.run("main:app", host="127.0.0.1", port=8000, reload=True)