New installations register themselves on first startup

Standing up a new MeshWeaver installation no longer involves copying access tokens between systems. A platform admin mints a reusable registration key once, puts it in the deployment scaffold, and every new installation registers itself at the plugin registry the first time it starts — receiving and safely storing its own access key automatically.

Combined with the default plugin grants introduced alongside, a brand-new installation reaches a filled Plugin Catalog with no manual steps: boot, self-register, see the platform plugins. Registration keys can expire and be revoked at any time from the Instance grants administration tab — revoking one stops future registrations without affecting installations already registered.