Fix web push subscription endpoint URL

[yissachar]

Summary

• Generate the Web Push subscription endpoint URL from the Rails route helper.
• Pass that URL to the browser in a meta tag so subscription POST and DELETE requests follow the engine mount and /_ route scope.

Problem

The browser derived /web_push/subscription from the service worker URL, while the engine route is /_/web_push/subscription. Enabling notifications therefore returned 404.

Verification

• git diff --check
• Tests not run.