Modernize Rails 8.1 app and consolidate ActiveStorage Uploadcare integration

[vipulnsward]

Summary

• modernize the example app to a Rails 8.1 baseline
  • run rails app:update and adopt current template/config updates
  • align Ruby toolchain and CI Ruby version to 4.0.1
  • update lint/security/test dependencies and generated scripts
• add and stabilize ActiveStorage example CRUD flow
  • "Using with Active Storage" page for upload/list/show/edit/delete
  • ActiveStorage model + migrations + request/spec coverage
• fix failing/fragile pages and request flows in files/posts/groups areas
• remove app-local ActiveStorage monkey patches and rely on uploadcare-rails gem integration
  • delete local previewer/variant overrides
  • keep an initializer spec asserting gem integration is active

Why

• keep the example app aligned with latest Rails defaults and runtime conventions
• avoid duplicated integration code between example app and gem
• ensure ActiveStorage + Uploadcare behavior is demonstrated through standard Rails APIs

Verification

• mise exec -- bundle exec rails zeitwerk:check
• mise exec -- bin/rubocop --parallel
• mise exec -- bundle exec rails_best_practices
• mise exec -- bundle exec brakeman
• mise exec -- bundle exec rspec

All checks pass locally.