#113 feat: add webhook signing secret rotation endpoint

This adds POST /webhooks/{webhook_id}/signing-secret/rotate, shipped in resend/resend-monorepo#10086. The response is the same shape as create, { object, id, signing_secret }, so callers get the new secret without a second GET. Rollout: https://linear.app/resend/issue/DEV-2070

Redocly lint on this branch vs main adds one warning, the missing 4XX response, which every sibling webhook operation already has.

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