security: remove secrets from git, gitignore k8s secret files

Secrets committed in prior commits (matomo-secret.yaml,
secret-analytics-auth.yaml) have been scrubbed from history via
filter-branch — rotate those credentials immediately.

Going forward:
- deploy/*-secret.yaml is gitignored; apply manually with kubectl
- deploy/matomo-secret.yaml.example shows the required shape
- ArgoCD ignoreDifferences on matomo-secret prevents it pruning a
  manually-applied secret
- Remove matomo-secret.yaml from kustomization.yaml so ArgoCD never
  manages it again

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

argocd/application.yaml

@@ -12,6 +12,14 @@ spec:
   destination:
     server: https://kubernetes.default.svc
     namespace: solitaire
+  # Secrets are applied manually and must not be pruned by ArgoCD.
+  ignoreDifferences:
+    - group: ""
+      kind: Secret
+      name: matomo-secret
+      namespace: solitaire
+      jsonPointers:
+        - /data
   syncPolicy:
     automated:
       prune: true