- Gemfile: bump to rails ~> 7.1.6, keep next_rails for future hops
- config/application.rb: load_defaults 7.1, add_autoload_paths_to_load_path = false
- config/environments/{dev,test,prod}.rb: cache_classes -> enable_reloading
- spec/rails_helper.rb: fixture_path -> fixture_paths (plural, 7.1 API)

Process: dual-booted with next_rails, ran breaking-change detection against
rails-upgrade skill's 7.1 patterns, fixed each with NextRails.next? branches
under Gemfile.next, then consolidated once 7.1 went green. load_defaults 7.1
flags flipped via the rails-load-defaults skill.

Not flipped: action_view.sanitizer_vendor, action_text.sanitizer_vendor.
Rails::HTML constant isn't loaded at initializer time because config/application.rb
uses selective railtie loading (no action_text, no active_storage). App doesn't
use sanitize/strip_tags/Action Text, so harmless. load_defaults 7.1 applies the
internal defaults via a different code path that doesn't fail.

Tailwind: move app/assets/stylesheets/application.tailwind.css to
app/assets/tailwind/application.css and rewrite for Tailwind v4
(@import "tailwindcss", @reference in component layer). Disclaimer: this
was already broken before the Rails upgrade — tailwindcss-rails 4.4.0 (which
ships Tailwind v4) was already in Gemfile.lock on the base branch, but the
CSS file was still using v3 syntax and the v3 default path. The Rails upgrade
did not cause this; it only surfaced during the smoke test.

Tests: 5/5 passing on Rails 7.1.6. Only residual deprecation is
Rails.application.secrets coming from a dependency's internals — flagged for
the 7.1 -> 7.2 hop.