feat(adr-075): 補全 2 個欠缺的 Prometheus 告警規則

- MomoScraperSuccessLow: 業務爬蟲成功率 <90% (business group)
- CoreDNSResolutionFailed: CoreDNS SERVFAIL 率 >5% (kubernetes group)

ADR-075 Phase 3 完成

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

ops/monitoring/alerts-unified.yml

@@ -175,6 +175,22 @@ groups:
           summary: "188 Host 備份超過 25 小時未成功"
           description: "backup-from-110.sh 可能失敗，/backup/110 資料可能過舊"
 
+      # ADR-075: CoreDNS 解析失敗 (2026-04-12 ogt)
+      - alert: CoreDNSResolutionFailed
+        expr: rate(coredns_dns_responses_total{rcode="SERVFAIL"}[5m]) > 0.05
+        for: 5m
+        labels:
+          severity: critical
+          layer: k8s
+          team: ops
+          auto_repair: "true"
+          alert_category: kubernetes
+          notification_type: TYPE-3
+        annotations:
+          summary: "CoreDNS SERVFAIL 率過高 {{ $value | humanizePercentage }}"
+          description: "CoreDNS 在 5 分鐘內 SERVFAIL 回應率超過 5%，K8s 服務間 DNS 解析可能失敗"
+          runbook: "kubectl -n kube-system get pods -l k8s-app=kube-dns && kubectl -n kube-system logs -l k8s-app=kube-dns"
+
   # =========================================================================
   # 資料庫告警 (database_alerts)
   # =========================================================================
@@ -711,6 +727,21 @@ groups:
         annotations:
           summary: "Gemini API 錯誤率過高: {{ $value | humanizePercentage }}"
           description: "Gemini API 5 分鐘錯誤率超過 20%，AI 降級可能失效"
+      # ADR-075: 業務爬蟲健康 (2026-04-12 ogt)
+      - alert: MomoScraperSuccessLow
+        expr: |
+          rate(momo_scraper_requests_total{status="success"}[5m])
+          / rate(momo_scraper_requests_total[5m]) < 0.9
+        for: 10m
+        labels:
+          severity: warning
+          layer: docker-110
+          auto_repair: "false"
+          alert_category: business
+          notification_type: TYPE-3
+        annotations:
+          summary: "Momo 抓取成功率跌至 {{ $value | humanizePercentage }}"
+          description: "Momo 爬蟲成功率低於 90%，業務資料可能缺失"
 
   - name: awoooi_flywheel_meta_alerts
     interval: 60s