fix(backend): quote WORKERS expansion next to fullwidth parentheses

Bash with set -u treated $WORKERS） as an unbound variable name.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/start.sh

@@ -163,7 +163,7 @@ start_server() {
     log_info "启动模式: 开发（热重载，前台）"
   else
     cmd+=(--workers "$WORKERS")
-    log_info "启动模式: 后台（workers=$WORKERS）"
+    log_info "启动模式: 后台（workers=${WORKERS}）"
   fi
 
   log_info "=========================================="