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>

deploy/kustomization.yaml

@@ -11,7 +11,6 @@ resources:
 - mariadb-deployment.yaml
 - mariadb-service.yaml
 - matomo-pvc.yaml
-- matomo-secret.yaml
 - matomo-deployment.yaml
 - matomo-service.yaml
 - ingress-analytics.yaml