fix: database ownership preserved during backup — app user must be able to write

- Backup now uses chown --reference to preserve original owner
- Found: rsync changed database ownership to root, blocking writes

install.sh

@@ -79,6 +79,8 @@ if [ "${1:-}" = "-update" ]; then
         sudo_if mkdir -p "$BACKUP_DIR"
         BACKUP_FILE="$BACKUP_DIR/expenses-$(date +%Y%m%d-%H%M%S).db"
         sudo_if cp "$INSTALL_DIR/expenses.db" "$BACKUP_FILE"
+        # Restore ownership so the app user can still write to the database.
+        sudo_if chown --reference="$INSTALL_DIR/expenses.db" "$BACKUP_FILE" 2>/dev/null || true
         info "Database backed up to $BACKUP_FILE"
     fi