feat(webhooks): expose webhook signing secret on creation

[ptaranat]

What changes I've made

• Add WebhookResourceWithSecret resource that extends WebhookResource to include the secret field
• Update both CreateWebhookAction and CreateOrganizerWebhookAction to use this new resource
• Update both CreateWebhookModal and CreateOrganizerWebhookModal to display the signing secret after creation with a copy-to-clipboard button and a warning that it won't be shown again
• Extract and compile translations for new UI strings

Why I've made these changes

Fixes #1136

The webhook signing secret is auto-generated on creation but never exposed to the user. This makes it impossible for webhook consumers on the hosted cloud platform to verify webhook signatures, defeating the purpose of the HMAC signing mechanism.

How I've tested these changes

• Created webhooks at both event and organizer level. Confirmed the signing secret modal appears after creation with a copy button
• Confirmed the secret is not returned on GET/list/edit endpoints
• Verified end-to-end webhook signature verification against a live staging receiver using the copied secret
• Backend unit tests pass (php artisan test --testsuite=Unit)
• No new TypeScript errors (tsc --noEmit)

Checklist

• I have read the contributing guidelines.
• My code follows the coding standards of the project.
• I have tested my changes, and they work as expected.
• I understand that this PR will be closed if I do not follow the contributor guidelines and if this PR template is left unedited.

[github-actions]

All contributors have signed the CLA ✍️ ✅
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[ptaranat]

I have read the CLA Document and I hereby sign the CLA

[daveearley]

Nice PR! Thank you.

Changes look good.