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