feat: add next_steps column and update session schemas

- Add next_steps TEXT column to sessions table via migration 034
- Add include_outcome_notes, include_next_steps, max_step_index to SessionExport
- Add next_steps to SessionUpdate, SessionResponse, SessionComplete

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/session.py

@@ -56,6 +56,9 @@ class Session(Base):
     scratchpad: Mapped[Optional[str]] = mapped_column(
         Text, nullable=True, server_default=sa.text("''")
     )
+    next_steps: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True, server_default=sa.text("''")
+    )
 
     # Relationships
     tree: Mapped["Tree"] = relationship("Tree", back_populates="sessions")