-- The Cover splits into two facts. `cover` keeps the third-party address the
-- bytes come from, which is what the acquisition path refetches and dedupes
-- on; `cover_address` is the content address of the bytes once they are
-- actually stored, and is what the wire's absolute URL is built from.
--
-- Empty `cover_address` therefore means "no Cover yet" rather than "a Cover
-- that 404s", which is the distinction the API and the UI both depend on.
ALTER TABLE series ADD COLUMN cover_address text NOT NULL DEFAULT '';