fix(l1): escalations queue orders by last_step_at (escalated_at column does not exist)

L1WalkSession has no escalated_at column (only started_at/last_step_at/resolved_at
+ escalation_reason[_category]). The /escalations endpoint and its test referenced
escalated_at, which would AttributeError at query time / TypeError at construction.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/test_l1_api_ai_build.py

@@ -141,7 +141,6 @@ async def test_escalations_lists_escalated_sessions_for_engineer(client: AsyncCl
         status="escalated",
         started_at=now,
         last_step_at=now,
-        escalated_at=now,
     )
     test_db.add(sess)
     await test_db.commit()