fix: add status_update to step_type CHECK constraint

The generate_status_update service inserted AISessionStep with
step_type='status_update' which violated the DB CHECK constraint,
causing a 500 error. Also fix incorrect field name confidence_score
(should be confidence_at_step) and remove nonexistent confidence_tier.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/flowpilot_engine.py

@@ -992,8 +992,7 @@ async def generate_status_update(
             "generated_content": raw_response.strip(),
             "client_name": client_name,
         },
-        confidence_score=1.0,
-        confidence_tier="high",
+        confidence_at_step=1.0,
     )
     db.add(step)
     session.step_count += 1