Introduce ExchangeTokenAPIView at /api/account/exchange-token/ to
exchange temporary tokens for a DRF auth token and minimal user
profile (id, fullname, email, avatar). Tokens are one-time use and
validated via OnlineClassTokenManager. Added serializer, view, URL,
and Swagger examples.
Update CourseOnlineClassTokenAPIView to return a fixed join URL:
https://imamjavad.newhorizonco.uk/join-class?token={TOKEN}&slug={SLUG}.
Store course_slug in token payload and remove redirect_path and the
_base_components helper. Examples and docs updated.
Docs: add CHANGELOG_EXCHANGE_TOKEN.md and exchange_token_api.md;
update online_class_entry_flow.md.
BREAKING CHANGE: exchange-token endpoint moved from
/api/courses/auth/exchange-token/ to /api/account/exchange-token/.
Response shape changed: adds token, user.id is numeric, user.name
renamed to user.fullname, user.role and user.is_admin removed.
Online token response now returns a fixed URL and includes course
slug. redirect_path support removed; clients must use the returned
token for subsequent requests.
