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

app/api/lifespan.py

@@ -41,7 +41,7 @@ async def lifespan(app: FastAPI):
             connect_timeout=app_settings.validator_connect_timeout,
             max_concurrency=db_settings.get("default_concurrency", app_settings.validator_max_concurrency),
         )
-        if db_settings.get("validation_targets"):
+        if db_settings.get("validation_targets") is not None:
             validator.update_test_urls(db_settings["validation_targets"])
 
         # 验证 WorkerPool