fix(signoz_webhook): AIDataImpact.value 大寫 → .lower() 轉 DataImpact

AIDataImpact enum value 為 'NONE'/'READ_ONLY' 等大寫，
DataImpact enum value 為 'none'/'read_only' 等小寫，
轉換時補 .lower() 避免 ValueError。

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

apps/api/src/api/v1/signoz_webhook.py

@@ -320,7 +320,7 @@ async def create_signoz_approval(
                 affected_pods=ai_br.affected_pods,
                 estimated_downtime=ai_br.estimated_downtime,
                 related_services=ai_br.related_services,
-                data_impact=DataImpact(ai_br.data_impact.value),
+                data_impact=DataImpact(ai_br.data_impact.value.lower()),
             )
         else:
             blast_radius = BlastRadius(