Stop offering bucket-moved media for adding a second time.

Registration records a media's full public URL, so moving the object
inside the bucket changed its URL and both the bucket browser and the
add-path duplicate check stopped recognising it — letting the same
media be added again as a duplicate photo.

Match on the filename as well, which survives a move: such a file is
reported as moved (never selectable) and refused by the add path, so
the ws API and scripted adds are covered too. A filename shared by two
registered media is still treated as taken but carries no photo link,
so the status can never point at the wrong photo.