ci(obs): quote heredoc delimiter in release obs-secrets.env write

Same fix as nightly.yml: prevents shell expansion of '$' in secret
values after Gitea renders them. Keep in sync with nightly.yml.

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

.gitea/workflows/release.yml

@@ -107,7 +107,7 @@ jobs:
           mkdir -p /opt/familienarchiv/infra
           cp -r infra/observability /opt/familienarchiv/infra/
           cp docker-compose.observability.yml /opt/familienarchiv/
-          cat > /opt/familienarchiv/obs-secrets.env <<EOF
+          cat > /opt/familienarchiv/obs-secrets.env <<'EOF'
           GRAFANA_ADMIN_PASSWORD=${{ secrets.GRAFANA_ADMIN_PASSWORD }}
           GLITCHTIP_SECRET_KEY=${{ secrets.GLITCHTIP_SECRET_KEY }}
           POSTGRES_USER=archiv