Add a backend admin migration that creates a unique index on
lower(backend_users.email). Rows copied from WinterCMS may hold emails
that differ only in case, so the migration refuses to run and names the
clashing logins instead of choosing an account to drop.
- docs/backend: admin controllers, forms, lists and filters, relation
manager, users and permissions, settings, partials and widgets, admin SPA
- docs/services: storage, outbound HTTP, realtime, Web Push, search, parity
testing and the Frontend and AJAX (not provided) page
- Examples for cabana (with testdata/docs YAML), fetchguard, lighthouse and
its centrifugo driver, flare, beachcomber and typesense, tide; lighthouse
and beachcomber TestDocs* regions run on their Postgres harnesses
- concept map rows link their guide pages and the not-provided rows the
Frontend and AJAX page; index lists Backend, Database and Services
- TestDocsRequiredPages asserts the D-08 section order
