[examples] Next.js deletion webhook quickstart

[KayleeWilliams]

Why

examples/kitchen-sink is a 516-line scripted smoke test - useful for us, bouncy for evaluators. The first-time visitor from the marketing tweet needs a "working deletion handler in 5 minutes" story, not a test harness. This is the single highest-leverage onboarding artifact.

Scope

• New directory examples/nextjs-deletion-webhook
• Next.js App Router project with one API route that:
  • Receives the webhook
  • Verifies the signature with the new verifyWebhook() helper
  • Executes a mock deletion (deletes a row in a demo SQLite DB)
  • ACKs with the expected response shape
• README walking through setup in <= 5 steps
• Smoke test runs it in CI
• Deployable to Vercel with one click (or clear README instructions)

Priority: P1 - beta blocker
Depends on: verifyWebhook() helper