Feat/realtime webhook validation

[michaelvic123]

This PR adds:

• Real-time validation : Checks URL format and HTTPS requirement as user types, not just on blur
• HTTPS enforcement : Disallows HTTP URLs entirely
• Test Connection button : Lets users verify webhook URLs work before saving (requires backend mutation testWebhookUrl )
• Better error messages : Clear error messages for invalid/non-HTTPS URLs
• Updates timeout validation : Timeout field also shows errors in real-time
  Changes made:

1. CreateWebhookModal.tsx : Add test connection button, real-time validation
2. WebhookManager.tsx : Add real-time validation, test connection button
3. webhooks/[id]/page.tsx : Update timeout field to validate in real-time
4. django-backend/schema.py : Add test_webhook_url GraphQL mutation
5. webhook-components.test.tsx : Update test to match new error message
6. Fix linter warnings for unused variables

closes #573

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