- Drop UNIQUE(media_item_id,user_id,title): titles are display labels
shared verbatim across clients; same-title bookmarks on different pages
now coexist instead of 500ing (deleting over a tombstone no longer
blocks future creates with that title)
- Add origin_source column recording the creating client (android/web/
koreader), set once at insert, exposed in API responses
- Web reader auto-title mirrors KOReader's 'in <chapter>' convention,
falling back to 'Bookmark'; adds bookmark rename in the drawer