fix(alert): fingerprint 加 alertname 防跨告警指紋衝突 + 補入缺漏心跳分類

問題根因：
1. generate_fingerprint 用 alert_type（大量 alertname 落入 "custom"）
   → 不同告警名稱同目標共用指紋 → 30 分鐘 debounce 互相擋截
2. classify_alert_early 漏掉 DeadMansSwitch / NoAlertsReceived /
   PrometheusNotConnectedToAlertmanager → 落入 TYPE-3 一般告警

修復：
- alert_analyzer_service.py: 指紋改為 namespace:deployment:alertname:target_resource
  alertname 取自 labels（Alertmanager），fallback 到 alert_type（其他來源）
- incident_service.py: DeadMansSwitch → backup/TYPE-1；
  NoAlertsReceived + PrometheusNotConnectedToAlertmanager → alertchain_health/TYPE-8M
- 補 2 個測試，全套 627 passed

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

apps/api/tests/test_classify_alert_early.py

@@ -66,6 +66,12 @@ class TestInfoAlerts:
         ac, nt = classify_alert_early("Watchdog", "none", {})
         assert nt == "TYPE-1"
 
+    def test_deadmansswitch_heartbeat(self):
+        # DeadMansSwitch 心跳 → TYPE-1（補入 2026-04-12 ogt）
+        ac, nt = classify_alert_early("DeadMansSwitch", "warning", {})
+        assert ac == "backup"
+        assert nt == "TYPE-1"
+
     def test_backup_critical_not_type1(self):
         # critical backup 告警應走各自 prefix，不是純資訊
         ac, nt = classify_alert_early("BACKUP_MISSING", "critical", {})
@@ -115,6 +121,8 @@ class TestAlertchainHealth:
         "AlertChainBroken_Sentry",
         "NoAlertsReceived2Hours",
         "AlertChainUnhealthy",
+        "NoAlertsReceived",
+        "PrometheusNotConnectedToAlertmanager",
     ])
     def test_alertchain_alerts(self, alertname):
         ac, nt = classify_alert_early(alertname, "critical", {})