fix(monitoring): classify host memory pressure accurately

apps/api/src/services/decision_manager.py

@@ -960,6 +960,7 @@ async def _resolve_target_from_k8s(incident: "Incident", namespace: str) -> str
         # 並加 log 便於追蹤 fallback 路徑
         _ALERTNAME_KEYWORDS: dict[str, list[str]] = {
             "HostHighCpuLoad":        ["api", "web"],
+            "HostMemoryUsageHigh":    ["api", "web"],
             "HostOutOfMemory":        ["api", "web"],
             "DockerContainerUnhealthy": [],
             "DockerContainerExited":  [],
@@ -1064,7 +1065,7 @@ async def _fetch_metrics_snapshot(incident: Incident) -> dict:
         snapshots: dict = {}
 
         # 根據 alertname 選擇最相關的指標
-        if alertname in ("HostHighCpuLoad", "HostOutOfMemory"):
+        if alertname in ("HostHighCpuLoad", "HostMemoryUsageHigh", "HostOutOfMemory"):
             if instance:
                 host = instance.split(":")[0]
                 # P0 fix 2026-04-11: _instant_query 要求 dict，回傳 {"result": [...]}