docs: document the Admin UI

The admin console was completely undocumented — the README never mentioned
it exists, its URL, or that it starts automatically with npm start and
cannot be turned off.

Adds an Admin UI section (URL, the 7 tabs, upstream MCP hub role), an
ADMIN_TOKEN row in the config table, and a security note: the admin API
writes .env, so only port 3000 should ever be tunnelled.