400 on your Clerk webhook endpoint: Svix signature verification failed

# Symptom
POSTs from Clerk to your webhook endpoint return 400 ("Error occurred") and events show as failed in the dashboard.

# Confirm the cause
1. Verify against the RAW request body. Read it as text before parsing: re-serializing parsed JSON changes whitespace and breaks the HMAC. Read text, verify, then parse.
2. Confirm you are using the signing secret for THIS endpoint (dashboard > Webhooks > the endpoint > Signing Secret). It starts with whsec_ and every endpoint has its own.
3. Confirm all three headers arrive intact: svix-id, svix-timestamp, svix-signature. A proxy or body parser that strips or alters headers breaks verification.
4. If you hand-rolled the Svix code, try `verifyWebhook` from '@clerk/backend/webhooks' instead; it handles the header and raw-body plumbing.

# Fix
- Typical correct shape: read the raw text body, construct the verifier with the endpoint's whsec_ secret, verify with the three headers, then parse and route on the event type. Return 200 only after handling.

# Verify
You cannot test with curl or Postman; they cannot produce valid Svix signatures. Use the dashboard's Send Test Event button, or tunnel your dev server and trigger a real event (sign up a test user).