Migrate example app to Uploadcare 5-0-stable APIs

[vipulnsward]

Summary

• pin the example app to uploadcare-rails and uploadcare-ruby 5-0-stable branches
• migrate examples to the current 5.x API surface (client accessors, conversions, add-ons, metadata, webhooks)
• switch uploader UI usage to File Uploader Web Components across Post and Comment forms
• add Active Storage example model, routes, views, and request/system coverage
• expand request and system specs across examples and edge cases, plus README/menu cleanup

Verification

• rtk mise exec -- bundle exec rspec
• rtk mise exec -- bundle exec rubocop