Idea: Login for TV apps (OAuth 2.0 Device Authorization Grant)

## Problem

Enables devices with no browser or limited input capability to obtain an access token. This is commonly seen on Apple TV apps, or devices like hardware encoders that can stream video to a YouTube channel.

## Appetite

6 weeks

## Solution

The _OAuth 2.0 Device Authorization Grant_ specification is designed for that: https://datatracker.ietf.org/doc/html/rfc8628

- The device (client) request login from Authgear
- Authgear returns the code and URI and the device presents them to the end-user, preferably in QR code format
- The end user scan the QR code and open the AuthUI in browser
- Complete the authentication flow in browser
- Sees the device logged in. It obtains the access token and refresh token.

No files or tests are named. Start by reading RFC 8628 and tracing Authgear's existing authentication and token entry points; done means a limited-input device can receive a code and URI, the user can authenticate in AuthUI, and the device obtains access and refresh tokens.

Written by the indexing model from the issue text.