Orphan detection moved outside the media_items delete in the batched
library-delete rewrite, opening a TOCTOU race: a concurrent scan/import
could attach one of the collected content IDs to another library between
collectOrphanBatch and the delete, after which the unconditional
`DELETE FROM media_items WHERE content_id = ANY($1)` would still remove the
shared row and cascade away the newly-added membership — dropping the item
from the other library. Re-check the orphan invariant inside the delete
(NOT EXISTS a membership in another folder) and count rows actually deleted.
Addresses PR #21 review (P1).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
