Discovery for AI-translatable adapters.

A feature module opts a resource into AI translation by exporting ai_translatables/0 returning [{resource_type, adapter_module}], where adapter_module implements PhoenixKitAI.Translatable. This scans every module known to PhoenixKit.ModuleRegistry for that function (duck-typed — the function is not a PhoenixKit.Module callback, so feature modules declare AI-translatability without core knowing anything about AI).

resource_type strings must be globally unique; on a collision the first registered module wins.