Gettext backend for phoenix_kit_entities.

Owns the translation catalogues under priv/gettext/. Locale is set per-request by the parent application; this module is only responsible for looking msgids up against the active locale.

See guides/per-module-i18n.md in the phoenix_kit core guides for the full setup and conventions. In short: every module that ships Gettext-wrapped UI must own its own backend rather than reaching for PhoenixKitWeb.Gettext — that backend belongs to core and its catalogues live in core's own package, so strings extracted against it from here would never end up in any .po file this package ships.