Add FastAPI backend with agent system

backend/app/schemas/conversation.py

@@ -0,0 +1,45 @@
+from pydantic import BaseModel
+from datetime import datetime
+
+
+class MessageCreate(BaseModel):
+    content: str
+
+
+class MessageOut(BaseModel):
+    id: str
+    role: str
+    content: str
+    model: str | None
+    tokens_used: int | None
+    created_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ConversationCreate(BaseModel):
+    title: str | None = None
+
+
+class ConversationOut(BaseModel):
+    id: str
+    title: str | None
+    message_count: int
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}
+
+
+class ChatRequest(BaseModel):
+    message: str
+    conversation_id: str | None = None
+    agent_id: str | None = None
+    file_ids: list[str] = []  # 新增
+
+
+class ChatResponse(BaseModel):
+    conversation_id: str
+    message_id: str
+    content: str
+    agent_name: str