⬆️ Upgrade to Rails 8.1 and update gems

- Bump rails to ~> 8.1.3 and run app:update (adds bin/ci, config/ci.rb, refreshed error pages), restoring app customizations that were overwritten - Enable load_defaults 8.1 - Update all gems; relax sqlite3 and aws-sdk-s3 pins - Pin json to ~> 2.21 until Active Support supports json 3.x keyword-only parse options - Add Solid Queue 1.7 batches migration and regenerate queue schema - Add brakeman.ignore for the intentional presigned S3 redirect