Add an admin-set full feature access override for shopkeepers

[dadachi]

Summary

Ports MyTurnTag's feature_access_state override from myturntagapi so the paid iOS/Android clients can unlock every paid feature without a purchase: testing on production, staff/partner/comp accounts, and App Review demo accounts.

• shopkeepers.feature_access_state enum: normal_feature_access (1, default) / all_feature_access (2), null: false, indexed. Same names and values as myturntagapi.
• Editable only in /madmin (ShopkeeperResource). Not permitted in sign-up or PATCH /shopkeeper_auth.
• Sign-in (ShopkeeperSignInSerializer) returns feature_access_state and account_owner_feature_access_state; accounts (AccountSerializer) return owner_feature_access_state. The paid clients read these exact field names and string values. The accounts index already preloads owner, so this adds no queries.
• Fixtures: one is normal_feature_access, two is all_feature_access, so tests cover both values.
• Docs: OpenAPI schemas, README "Full feature access override" (when to use it, plus RevenueCat promotional entitlements as the no-code alternative: they need billing on and are keyed to the owner's app user ID), an AGENTS.md "Billing (paid clients)" section, and CHANGELOG.

Tests

• madmin_test: the shopkeeper edit form, submitted as the browser sends it, sets all_feature_access
• registrations_controller_test: sign-up and PATCH /shopkeeper_auth with feature_access_state: "all_feature_access" both leave it normal_feature_access
• Serializer tests: sign-in returns both fields; accounts return the owner's state, not the viewing member's

To confirm each new test can fail, I permitted the param in both devise sanitizers, removed the madmin attribute and hard-coded the serializer values. All 6 new tests went red, then green again once the code was restored.

Local results: bin/rails test (643 runs, 0 failures), bin/rubocop, erb_lint, brakeman, bundler-audit and RAILS_ENV=test db:seed_fu all pass. I also ran a dev server and set the override on a seeded shopkeeper. Sign-in and GET /accounts returned all_feature_access, and a PATCH /shopkeeper_auth that tried to change it was ignored.

🤖 Generated with Claude Code

https://claude.ai/code/session_016JZoj1UtkFVFNcwoDfdb9f