User feedback (Sokhi): after changing cover-art sources, re-tag should
re-download fresh covers from THEM. The job took cover_url only from the
matched metadata source's album image, ignoring the user's configured
cover-art order. Now prefer select_preferred_art_url (the same
metadata_enhancement.album_art_order the post-process embed honors), falling
back to the source image when no order is configured (non-breaking).
'replace' cover mode already force-refreshes art on every matched album, and
the embed replaces existing art (no duplicate pictures) — so 'replace' + a
configured art order = fresh covers from those sources. Help text updated.
Tests: prefers configured source URL / falls back to source image when unset.
93d2399cf8