DNS overrides verifiedWritten by the Hub

A DNS override assigns a fixed address to a hostname, and the engine uses it for every message to that host. It is the right tool for a receiver whose published records are wrong, for keeping test traffic inside a lab, and for pinning a destination while a migration is in progress. The screen exists and works, but it is not in the settings menu. It opens from a remote address card, which means an operator can only find it after mail has already gone somewhere — precisely the wrong order for a setting whose purpose is to decide where mail goes. For Hub the useful part is the table: dns is writable like any other, so overrides can be managed without the screen at all.