feat: exempt service accounts from PREVENT_CONCURRENT_LOGINS single-login enforcement

[blarghmatey]

Description

Adds an opt-in way to exempt specific accounts from PREVENT_CONCURRENT_LOGINS single-login enforcement.

enforce_single_login (common/djangoapps/student/models/user.py) deletes a user's previously registered session on every login (one session slot per user, via UserProfile.set_login_session). For human accounts that is the intended behavior. For shared service/automation accounts — most notably the xqueue-watcher grader account, where many concurrent workers all authenticate as one user — each login evicts the other workers' sessions, causing a self-sustaining login storm and persistent 401/403s that stall external grading.

This change lets operators exempt such accounts via two new settings, both defaulting to empty so existing behavior is unchanged unless configured:

• SINGLE_LOGIN_EXEMPT_USERNAMES — iterable of exact usernames
• SINGLE_LOGIN_EXEMPT_GROUPS — iterable of group names (membership in any exempts the user)

When a logging-in user matches, enforce_single_login returns early; PREVENT_CONCURRENT_LOGINS remains fully enforced for everyone else.

Fixes #38824

How was this tested?

Added test_single_session_exempt_user in openedx/core/djangoapps/user_authn/views/tests/test_login.py, asserting that with PREVENT_CONCURRENT_LOGINS enabled and the user in SINGLE_LOGIN_EXEMPT_USERNAMES, a second concurrent login does not record the single-session slot (and therefore does not evict the first session). The existing test_single_session continues to assert the default (non-exempt) eviction behavior.

Settings are backward compatible

Both settings read via getattr(..., None) or (), so deployments that don't define them behave exactly as before.

🤖 Generated with Claude Code

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