Add FastAPI backend with agent system

backend/app/models/conversation.py

@@ -0,0 +1,26 @@
+from sqlalchemy import Column, String, Text, Integer, ForeignKey, JSON
+from sqlalchemy.orm import relationship
+from app.models.base import BaseModel
+
+
+class Conversation(BaseModel):
+    __tablename__ = "conversations"
+
+    user_id = Column(String(36), ForeignKey("users.id"), nullable=False, index=True)
+    title = Column(String(500), nullable=True)
+    message_count = Column(Integer, default=0)
+
+    messages = relationship("Message", back_populates="conversation", cascade="all, delete-orphan")
+
+
+class Message(BaseModel):
+    __tablename__ = "messages"
+
+    conversation_id = Column(String(36), ForeignKey("conversations.id"), nullable=False, index=True)
+    role = Column(String(20), nullable=False)  # user, assistant, system
+    content = Column(Text, nullable=False)
+    model = Column(String(100), nullable=True)
+    tokens_used = Column(Integer, nullable=True)
+    attachments = Column(JSON, nullable=True)  # 新增: [{file_id, filename, file_type, file_size}]
+
+    conversation = relationship("Conversation", back_populates="messages")