feat(nightscout): migrate from bare-metal LXC to k3s

apps/nightscout/nightscout-secret.yaml.example

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+# DO NOT COMMIT THE REAL VERSION OF THIS FILE.
+# apps/nightscout/nightscout-secret.yaml is gitignored — apply it manually once:
+#
+#   cp apps/nightscout/nightscout-secret.yaml.example apps/nightscout/nightscout-secret.yaml
+#   # fill in real values below, then:
+#   kubectl apply -f apps/nightscout/nightscout-secret.yaml
+#   kubectl annotate secret nightscout-secret -n nightscout \
+#     argocd.argoproj.io/sync-options=Prune=false --overwrite
+#
+# API_SECRET must be at least 12 characters.
+apiVersion: v1
+kind: Secret
+metadata:
+  name: nightscout-secret
+  namespace: nightscout
+stringData:
+  API_SECRET: "CHANGE_ME"
+  MONGODB_URI: "mongodb://mongodb:27017/nightscout"