Upgrade Rails 8.0 -> 8.1

Why

Final step. Verified that rails 8.1.3.1 resolves cleanly against this gem set today.

Scope

• gem "rails", "~> 8.1.0", bin/rails app:update
• Framework defaults for 8.1 handled the same way as the previous two: commented first, flipped in a separate deploy if any affect serialization formats

Acceptance criteria

• Full suite green
• Staging soak before production

Blocked by

#887 (Rails 8.0 defaults).

Known remaining work after this sequence

• OmniAuth 1.9 -> 2.x / Rack 2 -> 3. Rails 8.1 resolves fine while staying on Rack 2.2.24, because omniauth 1.9.2 caps rack < 3 and actionpack 8.1 only needs rack >= 2.2.4. So this does not block the upgrade — but it leaves the app on omniauth 1.x, which carries CVE-2015-9284 (authentication request forgery). Fixing it changes the auth request from GET to POST with a CSRF token, which breaks both the Ember and native clients until they are updated. Deserves its own issue and its own schedule.
• AMS 0.9.8 -> 0.10.x, or off AMS entirely. The pin is a deliberate freeze, not a fix.
• PostgreSQL 12.8 and MongoDB 4.4.9 are both past end of life in docker-compose.yml and CI.

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 blocked issue #887, then review the Rails version constraint and run bin/rails app:update. Check the 8.1 framework defaults and how they affect serialization formats; done means the full suite is green and staging has completed its soak before production.