Add FastAPI backend with agent system

backend/app/models/folder.py

@@ -0,0 +1,13 @@
+from sqlalchemy import Column, String, ForeignKey, UniqueConstraint
+from app.models.base import BaseModel
+
+
+class Folder(BaseModel):
+    __tablename__ = "folders"
+    __table_args__ = (
+        UniqueConstraint('user_id', 'parent_id', 'name', name='uq_user_parent_name'),
+    )
+
+    user_id = Column(String(36), ForeignKey("users.id"), nullable=False, index=True)
+    name = Column(String(255), nullable=False)
+    parent_id = Column(String(36), ForeignKey("folders.id"), nullable=True)