feat: add export and backup commands

- nib export: dump all entities to JSON (stdout or --output file)
- nib backup: atomic SQLite backup via VACUUM INTO (WAL-safe)
- Store.Backup() method on db layer
- Tests for both commands

internal/db/db.go

@@ -51,6 +51,11 @@ func (s *Store) Close() error {
 	return s.db.Close()
 }
 
+func (s *Store) Backup(dst string) error {
+	_, err := s.db.Exec("VACUUM INTO ?", dst)
+	return err
+}
+
 const currentSchema = 4
 
 var migrations = []func(db *sql.DB) error{