feat: 初始化孕育无忧全栈项目基础代码结构

完成项目初始搭建，包含后端FastAPI+MongoDB服务、前端React+Vite项目的完整基础结构，添加各类配置文件与依赖清单，涵盖核心业务初始框架。

backend/app/models/chat.py

@@ -0,0 +1,24 @@
+from datetime import datetime, timezone
+
+from beanie import Document, PydanticObjectId
+from pydantic import BaseModel, Field
+
+
+class ChatMessage(BaseModel):
+    role: str  # user / assistant
+    content: str
+    timestamp: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+
+
+class ChatSession(Document):
+    user_id: PydanticObjectId
+    report_id: PydanticObjectId
+    messages: list[ChatMessage] = Field(default_factory=list)
+    created_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+    updated_at: datetime = Field(default_factory=lambda: datetime.now(timezone.utc))
+
+    class Settings:
+        name = "chat_sessions"
+        indexes = [
+            [("user_id", 1), ("report_id", 1)],
+        ]