Move stats publishing from the dev bucket to prod

[Zethson]

The deploy workflow publishes to scverse-stats-dev because the AWS_ROLE_ARN secret holds GitHubActions-Stats-Manager-dev, whose IAM policy only covers the dev bucket — syncing to scverse-stats fails on s3:ListBucket (#12).

To move to prod:

• Confirm the prod role exists: aws iam get-role --role-name GitHubActions-Stats-Manager. If not, apply prod in scverse/aws: tofu init -backend-config=conf/backend-prod.conf -reconfigure && tofu apply -var-file=conf/prod.tfvars
• Repoint the secret: gh secret set AWS_ROLE_ARN --repo scverse/stats --body "arn:aws:iam::<account>:role/GitHubActions-Stats-Manager"
• Set STATS_BUCKET: scverse-stats in .github/workflows/deploy.yaml
• Update the three fetches in scverse/scverse.github.io (layouts/stats/list.html, layouts/people/list.html, layouts/shortcodes/stats.html) to the prod bucket