feat: save next_steps on session completion and update

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

backend/app/api/endpoints/sessions.py

@@ -236,6 +236,7 @@ async def complete_session(
     session.completed_at = datetime.now(timezone.utc)
     session.outcome = completion_data.outcome
     session.outcome_notes = completion_data.outcome_notes
+    session.next_steps = completion_data.next_steps
     await db.commit()
     await db.refresh(session)
     return session