fix(P0): classify_alert_early 參數名稱修正 _labels→labels

webhooks.py 呼叫傳 labels= 但函數定義用 _labels，導致所有
Alertmanager webhook 500，告警鏈路完全中斷。

2026-04-12 ogt
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/api/src/services/incident_service.py

@@ -101,7 +101,7 @@ def extract_affected_services(labels: dict, target_resource: str) -> list[str]:
     return []
 
 
-def classify_alert_early(alertname: str, severity: str, _labels: dict) -> tuple[str, str]:
+def classify_alert_early(alertname: str, severity: str, labels: dict | None = None) -> tuple[str, str]:
     """
     ADR-073 Phase 2-2: 早期分診，在 LLM 分析前決定 alert_category + notification_type。
     防止 HostBackupFailed 等告警被誤路由到 K8s executor。