Add FastAPI backend with agent system

backend/app/models/agent.py

@@ -0,0 +1,28 @@
+from sqlalchemy import Column, String, Text, Boolean, ForeignKey, Integer
+from sqlalchemy.orm import relationship
+from app.models.base import BaseModel
+
+
+class Agent(BaseModel):
+    __tablename__ = "agents"
+
+    name = Column(String(100), nullable=False)
+    role = Column(String(100), nullable=False)  # master, planner, executor, librarian, analyst
+    description = Column(Text, nullable=True)
+    system_prompt = Column(Text, nullable=False)
+    is_active = Column(Boolean, default=True)
+    is_default = Column(Boolean, default=False)
+
+    messages = relationship("AgentMessage", back_populates="agent", cascade="all, delete-orphan")
+    replies = relationship("ForumReply", back_populates="agent")
+
+
+class AgentMessage(BaseModel):
+    __tablename__ = "agent_messages"
+
+    agent_id = Column(String(36), ForeignKey("agents.id"), nullable=False, index=True)
+    conversation_id = Column(String(36), ForeignKey("conversations.id"), nullable=False, index=True)
+    role = Column(String(20), nullable=False)  # system, user, assistant
+    content = Column(Text, nullable=False)
+
+    agent = relationship("Agent", back_populates="messages")