Create Auth Session

**Typical flow:** 1. Create a connection in the portal (or via `POST /api/v1/connections`) and note its `connectionId`. 2. When your user is ready to connect QuickBooks, call this endpoint with that `connectionId`. 3. Redirect or link your user to the returned `authFlowUrl` — they complete the QWC setup from there. 4. Poll `GET /api/v1/qwc-auth-setup/{connectionId}/status/authenticated` until `isConnected = true`. **connectionId** — accepts the prefixed ID returned by `GET /api/v1/connections`, a raw GUID, or your configured `externalId`. **redirectUrl** (optional) — where to send the user after the QWC setup page completes. **linkExpiryMins** (optional) — how long the auth setup URL remains valid, in minutes. Defaults to 30 minutes. Maximum allowed value is 10,080 minutes (7 days).