monitoring: HomelabPrometheusTargetDown + HomelabDiskCritical

Schliesst die zwei in ALERT_RULES.md identifizierten Hoch-Luecken:
- up==0 (5m) als critical in neuer Gruppe homelab-meta — Scrape-Targets
  (node-exporter/cadvisor/blackbox/traefik) sind nicht laenger stille
  Ausfaelle.
- Disk-Critical bei >95% (5m) als critical, zusaetzlich zum bestehenden
  Warning bei >85% — fuer DB/appdata/Cache-Schreibblockaden.

ALERT_RULES.md Tabellen und Status-Abschnitt aktualisiert.
Wird wirksam nach Prometheus-Reload via Komodo-Redeploy des monitoring-Stacks.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

monitoring/prometheus/alerts.yml

@@ -57,6 +57,15 @@ groups:
           summary: "Disk usage high on {{ $labels.mountpoint }}"
           description: "{{ $labels.mountpoint }} is above 85% used."
 
+      - alert: HomelabDiskCritical
+        expr: 100 * (1 - node_filesystem_avail_bytes{fstype!~"tmpfs|overlay"} / node_filesystem_size_bytes{fstype!~"tmpfs|overlay"}) > 95
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Disk critically full on {{ $labels.mountpoint }}"
+          description: "{{ $labels.mountpoint }} is above 95% used. Writes may start to fail (DB, appdata, cache)."
+
       - alert: HomelabHighMemoryUsage
         expr: 100 * (1 - node_memory_MemAvailable_bytes / node_memory_MemTotal_bytes) > 90
         for: 10m
@@ -130,3 +139,14 @@ groups:
         annotations:
           summary: "Critical container is down: {{ $labels.name }}"
           description: "The host textfile exporter reports that critical container {{ $labels.name }} is not running."
+
+  - name: homelab-meta
+    rules:
+      - alert: HomelabPrometheusTargetDown
+        expr: up == 0
+        for: 5m
+        labels:
+          severity: critical
+        annotations:
+          summary: "Prometheus target down: {{ $labels.job }} / {{ $labels.instance }}"
+          description: "Scrape target {{ $labels.instance }} (job {{ $labels.job }}) is unreachable. Metrics from this target are silent — alerts built on them will not fire."