fix: unify backend port to 18080 and make validator targets configurable

- Set default API port to 18080 in config.py
- Add configurable validation_targets to SettingsSchema and DEFAULT_SETTINGS
- Update ValidatorService to support runtime test URL updates
- Hot-reload validation_targets from DB on startup and on settings save
- Add domestic fallback URLs (baidu.com, qq.com) to reduce foreign dependency risk
- Update Settings.vue to allow adding/removing validator target URLs in UI

app/api/lifespan.py

@@ -41,6 +41,8 @@ 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"):
+            validator.update_test_urls(db_settings["validation_targets"])
 
         # 验证 WorkerPool
         async def validation_handler(proxy):