-- The Cover splits into two facts. `cover` keeps the third-party address the
-- bytes come from, which is what the refetch path 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.
--
-- The content address was originally the hex SHA-256 of the source URL
-- (ADR-0007). Since ADR-0014 it is the hex SHA-256 of the bytes themselves,
-- so a re-art behind the same URL is a new address. Rows written before
-- ADR-0014 keep their URL-derived addresses; they are never rehashed and heal
-- into byte addressing on their first forced replacement. Both derivations
-- share the 64-hex-digit shape, so the serving guard is unchanged.
ALTER TABLE series ADD COLUMN cover_address text NOT NULL DEFAULT '';