feat(disk-usage-alert): Telegram alert when turing3 SSD >=90%

The SSD on turing3 (/mnt/ssd) is shared by Postgres, Plex's SQLite DB
and the media library. When it fills, Postgres crashes (cannot write
postmaster.pid) and Plex's library DB corrupts. Add a CronJob that
checks df via the plex-data mount every 15m and warns Telegram at >=90%
used (3h cooldown). Bot token/chatId live in the out-of-band
`telegram-disk-alert` Secret, not in git.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

applications/disk-usage-alert.yaml

@@ -0,0 +1,23 @@
+apiVersion: argoproj.io/v1alpha1
+kind: Application
+metadata:
+  name: disk-usage-alert
+  namespace: argocd
+spec:
+  project: default
+  source:
+    repoURL: http://gitea-http.gitea.svc.cluster.local:3000/admin/turingpi.git
+    targetRevision: HEAD
+    path: manifests/disk-usage-alert
+    directory:
+      recurse: false
+  destination:
+    server: https://kubernetes.default.svc
+    namespace: default
+  syncPolicy:
+    automated:
+      prune: true
+      selfHeal: true
+    syncOptions:
+    - CreateNamespace=true
+    - ServerSideApply=true

manifests/disk-usage-alert/cronjob.yaml

@@ -0,0 +1,81 @@
+# SSD disk-usage alert for turing3 /mnt/ssd (shared by Postgres + Plex + media).
+# A full SSD crashes Postgres and corrupts Plex's SQLite DB, so we warn early.
+#
+# NOTE: the Telegram bot token + chat id live in the `telegram-disk-alert` Secret,
+# created out-of-band (NOT in git) so the token stays out of history:
+#   kubectl -n default create secret generic telegram-disk-alert \
+#     --from-literal=botToken='<token>' --from-literal=chatId='<chatId>'
+#
+# Mounts the existing RWX plex-data PVC purely to read `df` of the underlying SSD.
+apiVersion: batch/v1
+kind: CronJob
+metadata:
+  name: disk-usage-alert
+  namespace: default
+spec:
+  schedule: "*/15 * * * *"
+  concurrencyPolicy: Forbid
+  successfulJobsHistoryLimit: 1
+  failedJobsHistoryLimit: 3
+  jobTemplate:
+    spec:
+      backoffLimit: 0
+      activeDeadlineSeconds: 120
+      template:
+        spec:
+          restartPolicy: Never
+          containers:
+            - name: check
+              image: curlimages/curl:8.11.1
+              imagePullPolicy: IfNotPresent
+              env:
+                - name: THRESHOLD          # alert at/above this % used
+                  value: "90"
+                - name: COOLDOWN_SEC       # min seconds between alerts (3h)
+                  value: "10800"
+                - name: MOUNT
+                  value: "/data"
+                - name: BOT_TOKEN
+                  valueFrom:
+                    secretKeyRef: { name: telegram-disk-alert, key: botToken }
+                - name: CHAT_ID
+                  valueFrom:
+                    secretKeyRef: { name: telegram-disk-alert, key: chatId }
+              command: ["/bin/sh", "-c"]
+              args:
+                - |
+                  set -u
+                  pct=$(df -P "$MOUNT" | awk 'END{gsub("%","",$5); print $5}')
+                  avail=$(df -Ph "$MOUNT" | awk 'END{print $4}')
+                  now=$(date +%s)
+                  marker="$MOUNT/.disk-alert-last"
+                  send() {
+                    curl -s -m 15 "https://api.telegram.org/bot${BOT_TOKEN}/sendMessage" \
+                      --data-urlencode "chat_id=${CHAT_ID}" \
+                      --data-urlencode "text=$1" -d "parse_mode=HTML" >/dev/null
+                  }
+                  if [ "${TEST:-0}" = "1" ]; then
+                    send "✅ <b>turingpi disk-alert test</b> — SSD at ${pct}% (free ${avail}). Alerting works."
+                    echo "TEST sent (${pct}% used)"; exit 0
+                  fi
+                  if [ "$pct" -ge "$THRESHOLD" ]; then
+                    last=0; [ -f "$marker" ] && last=$(cat "$marker" 2>/dev/null || echo 0)
+                    if [ $((now - last)) -ge "$COOLDOWN_SEC" ]; then
+                      send "⚠️ <b>turingpi SSD ${pct}% full</b> — only ${avail} free on /mnt/ssd. Postgres + Plex crash at 100%. Prune content / check maintainerr."
+                      echo "$now" > "$marker"
+                      echo "ALERT sent (${pct}% >= ${THRESHOLD}%)"
+                    else
+                      echo "over threshold (${pct}%) but within cooldown; skipping"
+                    fi
+                  else
+                    echo "ok: ${pct}% used, ${avail} free (threshold ${THRESHOLD}%)"
+                  fi
+              resources:
+                requests: { cpu: 10m, memory: 16Mi }
+                limits: { memory: 64Mi }
+              volumeMounts:
+                - { name: data, mountPath: /data }
+          volumes:
+            - name: data
+              persistentVolumeClaim:
+                claimName: plex-data