Enable the Rails 8.0 framework defaults

Why

Same reasoning as the 7.2 defaults issue: framework defaults that change cached-value or signed-message formats must be deployed separately from the framework bump, or old and new processes cannot read each other's data during a rolling deploy.

Scope

• Uncomment the options in new_framework_defaults_8_0.rb one group at a time
• Bump config.load_defaults to 8.0 and delete the file once all are enabled

Check each option for serialization-format impact. If none apply, this folds back into the Rails 8.0 issue.

Acceptance criteria

• Full suite green
• load_defaults 8.0, defaults file deleted
• Deployed separately from the framework bump

Blocked by

#886 (Rails 8.0), deployed and soaked. Blocks #888.

Part of the Ruby + Rails upgrade sequence. Ruby 3.2 reached end of life on 2026-04-01 and the backend is on Rails 7.1; the goal is Ruby 3.4.10 and Rails 8.1, in steps that each keep CI green and are independently deployable.

Start with config/initializers/new_framework_defaults_8_0.rb and review each commented option one group at a time, checking whether it changes cached-value or signed-message serialization. Then inspect the Rails 8.0 configuration path and the dependency on #886. Done means all applicable options are enabled, config.load_defaults is 8.0, the defaults file is deleted, and the full suite is green.