[alerter]feature: add WPUSH as a new alert notification channel

[anhao]

What's changed?

Add WPUSH (https://wpush.cn) as a new alert notice receiver channel (type 16), following the ServerChan / Gotify / Ntfy pattern.

• Handler: WpushAlertNotifyHandlerImpl — POST https://api.wpush.cn/api/v1/send
• Success when HTTP 200 and JSON code == 0 (HTTP 2xx alone is not enough)
• Receiver fields: wpushToken (masked), optional wpushChannel / wpushTopicCode
• Preset template 16-WpushTemplate.txt, frontend type option + i18n (en/zh-CN/zh-TW/ja/ko/pt-BR)
• Checkstyle: class names use Wpush* (AbbreviationAsWordInName)

Checklist

• I have read the Contributing Guide
• I have written the necessary doc or comment.
• I have added the necessary unit tests and all cases have passed.

Local verification (OpenJDK 25):

./mvnw -pl hertzbeat-alerter -am test -Dtest=WpushAlertNotifyHandlerImplTest -Dsurefire.failIfNoSpecifiedTests=false

Result: Tests run: 5, Failures: 0, Errors: 0 (BUILD SUCCESS), including checkstyle.

Add or update API

• I have added the necessary e2e tests and all cases have passed.