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).

A host application that is not a kit module joins the same pipeline through config:

config :phoenix_kit_ai, translatables: [{"product", MyApp.AI.ProductTranslatable}]

resource_type strings must be globally unique; on a collision the first registered module wins — configured entries come first, so a host can also override a module's adapter for a type.