Cover addresses derived from the bytes; ReplaceSeriesCover (#150)

# Conflicts:
#	backend/internal/store/store_test.go

backend/internal/store/migrations/0009_series_cover_address.sql

@@ -1,8 +1,15 @@
 -- 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.
+-- 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 '';