Add WPush notification preset

[greyair]

Summary

Add WPush (wpush.cn) as a built-in notification preset alongside Custom Webhook and Telegram.

This provides another notification delivery option while reusing Uptimer's existing event, template, timeout, and delivery-deduplication infrastructure.

Changes

• add WPush channel configuration and validation
• support API keys encrypted in D1 or referenced from a Worker Secret
• send WPush messages through its form-encoded /api/v1/send API
• sanitize API key material from Admin API responses
• add WPush configuration UI with channel, option, URL, templates, timeout, and event filtering
• add worker tests for delivery success/failure and secret handling
• add admin tests for encrypted API key storage and edit preservation
• document WPush configuration in English and Simplified Chinese

Test Plan

• Worker delivery tests pass on the extracted implementation
• Manually tested in a deployed fork with a real WPush API key
• Upstream PR CI: pnpm lint, pnpm typecheck, pnpm test

Related Issues

Refs #76