Add support for Vercel webhooks

[Prateek32177]

We want to add webhook support for Vercel in tern. Vercel sends events for deployments, domains, projects, and more.

Tasks

Webhook Handler

• Implement a handler in /src/platforms/vercel.ts.
• Normalize payloads into tern’s event structure (id, type, timestamp, payload).

### Platform Config

Add platformConfigs.vercel with supported event types (e.g., deployment.created, deployment.ready, deployment.error).

Test Cases

Add fixtures for at least 3 event types (deployment ready, error, domain added).

Write Jest tests in /tests/vercel.test.ts to validate parsing.

Docs

Add setup instructions in /docs/platforms.md.

Example: how to configure a webhook in Vercel dashboard.

Acceptance Criteria

• Vercel handler implemented
• Tests cover 3+ event types
• Docs updated

Final Integration Testing

To complete a new platform integration in Tern, please ensure the signature verification works correctly:

• Trigger a webhook event from the platform’s dashboard (use the “Send Test Event” option if available).
• Verify the signature using the platform’s signing secret / HMAC.
• The event should pass verification if the signature is correct.
• The event should fail verification if the signature is invalid.

Add a test case in Jest covering both valid and invalid signature scenarios.

This ensures that the integration reliably validates incoming webhook events before processing.

Helpful Links: Vercel Webhooks Docs

[rohith-0601]

can work on this,assign it to me