fix: replace wget --spider with wget -qO /dev/null for healthcheck

--spider sends a HEAD request which newer BusyBox builds in nginx:alpine
may not handle consistently. A plain GET to /dev/null is more reliable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flight-comparator/Dockerfile.frontend

@@ -15,5 +15,5 @@ COPY nginx.conf /etc/nginx/conf.d/default.conf
 
 EXPOSE 80
 
-HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+HEALTHCHECK --interval=30s --timeout=5s --start-period=15s --retries=3 \
-    CMD wget -q --spider http://localhost/ || exit 1
+    CMD wget -qO /dev/null http://localhost/ || exit 1