fix(webhooks): alertmanager handler — alert_type 超範圍改為 custom

AlertPayload.alert_type 只接受 8 個 Literal 值
ALERTNAME_TO_TYPE 映射回傳 host_cpu/backup_failure 等不在白名單 → ValidationError
修法：凡不在 Literal 白名單的 alert_type 一律 fallback 為 "custom"

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

apps/api/src/api/v1/webhooks.py

@@ -1156,7 +1156,14 @@ async def alertmanager_webhook(
             )
 
     # I1 整合 ADR-064 Rule Engine 2026-04-11: YAML 規則動態推斷，ALERTNAME_TO_TYPE 為 fallback
+    # 2026-04-12 ogt: AlertPayload.alert_type 只接受 8 個 Literal，超範圍的改為 "custom"
+    _ALERT_TYPE_ALLOWED = {
+        "k8s_node_failure", "k8s_pod_crash", "db_connection_timeout", "service_404",
+        "high_cpu", "high_memory", "disk_full", "ssl_expiry", "custom",
+    }
     alert_type = get_incident_type(alertname)
+    if alert_type not in _ALERT_TYPE_ALLOWED:
+        alert_type = "custom"
 
     # ADR-073 Phase 2-2: 早期分診 — 在 LLM 前決定 alert_category + notification_type
     # 2026-04-12 ogt: 防止 HostBackupFailed 等被誤路由到 K8s executor