Add FastAPI backend with agent system

backend/app/models/knowledge_graph.py

@@ -0,0 +1,32 @@
+from sqlalchemy import Column, String, Text, Integer, Float, ForeignKey, JSON
+from sqlalchemy.orm import relationship
+from app.models.base import BaseModel
+
+
+class KGNode(BaseModel):
+    __tablename__ = "kg_nodes"
+
+    user_id = Column(String(36), ForeignKey("users.id"), nullable=False, index=True)
+    name = Column(String(500), nullable=False)
+    entity_type = Column(String(100), nullable=False)  # person, concept, task, document, chunk, tag
+    description = Column(Text, nullable=True)
+    properties_ = Column(JSON, nullable=True)  # 额外属性
+    source_document_id = Column(String(36), ForeignKey("documents.id"), nullable=True)
+    importance = Column(Float, default=0.5)  # 重要性 0-1
+    last_updated_by = Column(String(36), nullable=True)  # 哪个 agent 更新过
+
+    outgoing_edges = relationship("KGEdge", foreign_keys="KGEdge.source_id", back_populates="source_node", cascade="all, delete-orphan")
+    incoming_edges = relationship("KGEdge", foreign_keys="KGEdge.target_id", back_populates="target_node", cascade="all, delete-orphan")
+
+
+class KGEdge(BaseModel):
+    __tablename__ = "kg_edges"
+
+    source_id = Column(String(36), ForeignKey("kg_nodes.id"), nullable=False, index=True)
+    target_id = Column(String(36), ForeignKey("kg_nodes.id"), nullable=False, index=True)
+    relation_type = Column(String(100), nullable=False)  # related_to, part_of, caused_by, depends_on, etc.
+    weight = Column(Float, default=0.5)  # 关系强度 0-1
+    properties_ = Column(JSON, nullable=True)
+
+    source_node = relationship("KGNode", foreign_keys=[source_id], back_populates="outgoing_edges")
+    target_node = relationship("KGNode", foreign_keys=[target_id], back_populates="incoming_edges")