初始化项目基础结构

添加了完整的前后端开发环境配置，包括：
- 创建 .gitignore 文件忽略本地生成目录和环境文件
- 添加详细的 README.md 开发指南文档
- 配置 backend 目录结构和 FastAPI 应用基础框架
- 实现应用配置管理、健康检查和状态接口
- 设置 uv 依赖管理和虚拟环境配置
- 完成 CORS 中间件配置支持前端开发联调

backend/app/__init__.py

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+"""Notes Agent AI Core."""

backend/app/config.py

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+import os
+from dataclasses import dataclass
+from functools import lru_cache
+
+
+@dataclass(frozen=True)
+class Settings:
+    """应用基础配置；正式环境可通过 APP_* 环境变量覆盖。"""
+
+    name: str
+    version: str
+    environment: str
+    host: str
+    port: int
+
+
+@lru_cache
+def get_settings() -> Settings:
+    return Settings(
+        name=os.getenv("APP_NAME", "Notes Agent AI Core"),
+        version=os.getenv("APP_VERSION", "0.1.0"),
+        environment=os.getenv("APP_ENVIRONMENT", "development"),
+        host=os.getenv("APP_HOST", "127.0.0.1"),
+        port=int(os.getenv("APP_PORT", "8000")),
+    )

backend/app/main.py

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+from fastapi import FastAPI
+from fastapi.middleware.cors import CORSMiddleware
+
+from app.config import get_settings
+from app.schemas import HealthResponse, ServiceStatusResponse
+
+settings = get_settings()
+
+app = FastAPI(
+    title=settings.name,
+    version=settings.version,
+    description="AI 笔记软件的本地 FastAPI 服务壳子。",
+)
+
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["http://127.0.0.1:5173", "http://localhost:5173"],
+    allow_credentials=True,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+@app.get("/health", response_model=HealthResponse, tags=["System"])
+async def health() -> HealthResponse:
+    return HealthResponse()
+
+
+@app.get("/api/status", response_model=ServiceStatusResponse, tags=["System"])
+async def service_status() -> ServiceStatusResponse:
+    return ServiceStatusResponse(
+        name=settings.name,
+        version=settings.version,
+        environment=settings.environment,
+    )

backend/app/schemas.py

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+from typing import Literal
+
+from pydantic import BaseModel
+
+
+class HealthResponse(BaseModel):
+    status: Literal["ok"] = "ok"
+
+
+class ServiceStatusResponse(HealthResponse):
+    name: str
+    version: str
+    environment: str