fix(datastore): backfill stuck "Loading..." titles to hostname on boot
The entities feature now falls back to URL hostname when pageMetadata.title
is empty, but only on the fly for items the user revisits. Existing
rows whose page never reported a real title would otherwise stay
title-less indefinitely, leaving cards rendering as blank or "Loading...".
Adds a one-shot migration that scans URL items with title='Loading...'
or empty/null and rewrites them to the URL hostname (matching the same
fallback the entities flow uses). Idempotent via the standard migrations
table.