feat: add scratchpad field to session model and schemas

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/session.py

@@ -1,7 +1,8 @@
 import uuid
 from datetime import datetime, timezone
 from typing import Optional, Any
-from sqlalchemy import String, DateTime, ForeignKey, Boolean
+from sqlalchemy import String, DateTime, ForeignKey, Boolean, Text
+import sqlalchemy as sa
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.core.database import Base
@@ -44,6 +45,9 @@ class Session(Base):
     ticket_number: Mapped[Optional[str]] = mapped_column(String(100), nullable=True)
     client_name: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
     exported: Mapped[bool] = mapped_column(Boolean, default=False)
+    scratchpad: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True, server_default=sa.text("''")
+    )
 
     # Relationships
     tree: Mapped["Tree"] = relationship("Tree", back_populates="sessions")