Control plane and tenant registry

The Lummet control plane launched as a standalone application with its own database, separate from every brand it manages.

• A tenant registry records each brand's host, status and cached capabilities.
• Each tenant gets its own access credential. The secret is encrypted before storage and shown once.
• Connection tests and health checks report whether each brand is reachable, with clear error categories.
• Switching the active tenant changes only the admin's session, never the brand's own login state.