feat(bookmarks): location identity, origin provenance, KOReader-style labels

- 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

internal/database/models.go

@@ -185,6 +185,7 @@ type MediaBookmarks struct {
 	ChapterReference   pgtype.Int4        `db:"chapter_reference" json:"chapter_reference"`
 	Deleted            pgtype.Bool        `db:"deleted" json:"deleted"`
 	DeletedAt          pgtype.Timestamptz `db:"deleted_at" json:"deleted_at"`
+	OriginSource       pgtype.Text        `db:"origin_source" json:"origin_source"`
 }
 
 type MediaHighlights struct {