feat: add GitHub Device Authorization Grant (CLI auth) to GithubLoginHandler

[Mukesh-M-Lohar]

Summary

Adds support for the GitHub Device Authorization Grant (RFC 8628) directly inside the existing GithubLoginHandler, enabling CLI / browserless authentication without a separate handler class.

This is particularly useful when building custom integrations like Model Context Protocol (MCP) servers, allowing headless CLI environments to securely authenticate and access the tasks API.

Also documents the FLOWER_GITHUB_OAUTH_DOMAIN environment variable which partially addresses #1308 (GitHub Enterprise Server support).

How to Test

Setup

Configure flowerconfig.py with GitHub OAuth details and start Flower:

celery flower --auth_provider="flower.views.auth.GithubLoginHandler" \
              --auth="your-email@example.com" \
              --oauth2_key="<client_id>" \
              --oauth2_secret="<client_secret>"

Steps

1. Request Device Code (POST):

   curl -X POST http://localhost:5555/login

   Expected Response: JSON containing device_code, user_code, verification_uri.

2. Authorize Device:
   Go to the verification_uri in your browser and enter the user_code.

3. Poll for Auth status (GET):
   Query the polling endpoint:

   curl -i "http://localhost:5555/login?device_code=<device_code>"

   • Before authorization: Returns 202 Accepted (authorization_pending).
   • After authorization: Returns 302 Found with Location: / and sets the session cookie.

Changes

flower/views/auth.py

• Added _OAUTH_DEVICE_CODE_URL class attribute
• Added post() — device flow step 1: requests device_code + user_code from GitHub, returns JSON
• Added _device_poll() — device flow step 2: polls for access token, returns 202 while pending, 302 on success
• Existing web OAuth flow (GET /login, GET /login?code=…) is unchanged

tests/unit/views/test_auth.py

• 4 new unit tests covering: device code request (POST), authorization pending (202), auth error (403), and successful auth with cookie + redirect (302)

docs/auth.rst

• Added GitHub Device Auth (CLI) subsection with config example and links to GitHub docs
• Documented FLOWER_GITHUB_OAUTH_DOMAIN environment variable in the GitHub OAuth section (relates to OAuth2 workflow does not work with Github Enterprise Servers.  #1308)

No breaking changes

The existing OAuth web flow behaviour is fully preserved.

[Mukesh-M-Lohar]

@auvipy could you review this PR

[Mukesh-M-Lohar]

Hi! I have implemented the GitHub Device Authorization Grant (CLI auth) flow and updated the unit tests/documentation.

All 10 unit tests are passing successfully locally. Could a maintainer please approve the workflow run so that the CI checks can verify? Thank you!