Upgrade Rails 7.1 -> 7.2

Why

First framework step. With the prep and the two blocker gems done, this should be close to mechanical. Verified to resolve cleanly.

Scope

• gem "rails", "~> 7.2.0"
• Keep config.load_defaults at 7.1. Run bin/rails app:update, take the generated new_framework_defaults_7_2.rb with every option still commented. Enabling them is a separate issue — upgrading the framework and changing its behaviour are two different changes, and some defaults alter serialization formats that must not flip in the same deploy.
• config/initializers/new_framework_defaults_7_1.rb can be retired: it is already fully worked through and load_defaults 7.1 is set, so it is bookkeeping.
• Rails.application.secrets is removed in 7.2 — already handled by the prep issue.

Note Rails 7.2 requires Ruby >= 3.1, so this lands fine on the current Ruby 3.2.3.

Acceptance criteria

• Full suite green with deprecations raising
• load_defaults still 7.1
• Staging soak before the defaults are flipped

Blocked by

#881 (Mongoid 9) and #882 (Globalize 7) — both hard resolution blockers. Blocks #884 and #885.

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 by reviewing the Rails dependency scope and the two blocker issues, #881 and #882. Run bin/rails app:update, keep config.load_defaults at 7.1, retain the generated 7.2 defaults commented, and retire config/initializers/new_framework_defaults_7_1.rb. Done means the full suite passes with deprecations raising and staging completes its soak.