feat(api): move title files on root change

Changing a movie's or series' root previously rewrote root_id and left
the files behind, so the §7.4 layout stopped describing the disk and
the root's policy applied to a library the files were not in. Series
had no root control at all.

All roots share one ZFS dataset, so the move is a rename of the title
folder into the new root, never a copy — hardlinks and the seeding
torrent survive it (§7.3). Disk first, row second: a destination that
already holds the folder is a 409, a failed rename leaves the row
unchanged, and a title with nothing on disk moves with no filesystem
work. media_files rows are rewritten in the same transaction as the
root_id, and a successful move triggers the §7.5 Jellyfin refresh.

Issue #228

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

DESIGN.md

@@ -488,6 +488,8 @@ Media kind first, hard audience boundary second, people nowhere.
 - **Release group is deliberately absent.** It is not a selection criterion and
   it makes filenames long enough to break a terminal.
 
+Changing a title's root relocates its title folder into the new root; roots are assumed to share one filesystem, so the move is a rename, never a copy.
+
 During transition, write into the existing roots so Jellyfin needs no
 reconfiguration and new content appears immediately. Radarr will not touch a
 folder it has no record of.