Docusign Callback

OAuth callback endpoint for DocuSign consent (called by frontend).

DocuSign redirects the admin back to the Integrations page, which POSTs the code/state here via XHR (so the auth cookie is sent same-site). Exchanges the code for a one-shot access token, calls /oauth/userinfo to capture the per-customer DocuSign user GUID + account info, and persists those to the credential row. After this, JWT Grant token minting works for the customer.