media.entry_id modelled ownership as one entry per file, but link_to_entry
reassigns it on every save. Embedding the same image in a second entry
therefore moved the claim, and deleting that second entry deleted the file
out from under the first — a broken image in the entry and a 404 on its
share link.
A join table replaces the column. Deleting an entry now removes only the
files no other entry embeds, and link_to_entry runs in a transaction so an
entry is never briefly attached to nothing while its share link is being
read.
