Fix webhook creation request parsing

[quappefeeder]

Fixes #86.

POST /api/webhooks is wrapped by withAuth, which passes the original request as event.request. The route tried to parse request.json() from an undefined variable, so valid authenticated webhook creation bodies were rejected as invalid JSON before validation or insert logic could run.

Changes:

• Parse the body from event.request.json().
• Add a regression test that exercises authenticated webhook creation and verifies the insert payload.

Local checks:

• git diff --check
• node --check src/app/api/webhooks/route.js
• node --check tests/api/webhooks-route.test.js

Full Vitest was not run locally because this environment has no npm/pnpm/node_modules available.

via ugig gig, ugig user liudong-design