Plugin registry is now gated to registered instances

The plugin registry no longer serves its catalog publicly. An installation registers with the registry and receives an instance token; the Plugin Catalog sends it on every request, and the registry answers only requests carrying a valid token.

For operators: on the registry, list the issued tokens under PluginCatalog:RegistryTokens; on each consuming installation, set PluginCatalog:RegistryToken (or Registries:N:Token per registry) to the token it was issued. A registry with no tokens configured keeps answering anonymously — the local-dev and test-stub mode — so nothing changes for development setups.