Round 4 fixes: scheduler DB save check, empty validation_targets, proxy list page fallback, scheduler frontend state sync, tests

app/api/routes/settings.py

@@ -50,7 +50,7 @@ async def save_settings(
         validator._init_timeout = request.validation_timeout
         validator._init_connect_timeout = request.validation_timeout
         validator._init_max_concurrency = request.default_concurrency
-        if request.validation_targets:
+        if request.validation_targets is not None:
             validator.update_test_urls(request.validation_targets)
         # 先关闭现有 session，再重置 semaphore，避免竞态窗口
         await validator.close()