fix: update mode backs up database before making changes

- Creates timestamped backup at /opt/receiptnext/backups/expenses-YYYYMMDD-HHMMSS.db
- rsync only targets templates/ and static/ — database is never touched

install.sh

@@ -68,6 +68,15 @@ if [ "${1:-}" = "-update" ]; then
         cd "$INSTALL_DIR"
     fi
 
+    # Backup the database before making any changes.
+    if [ -f "$INSTALL_DIR/expenses.db" ]; then
+        BACKUP_DIR="$INSTALL_DIR/backups"
+        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"
+        info "Database backed up to $BACKUP_FILE"
+    fi
+
     # Copy updated templates, static assets, and config files.
     info "Updating templates and static assets..."
     sudo_if rsync -a --delete templates/ "$INSTALL_DIR/templates/" 2>/dev/null || true