Adds an optional roster.json (set of allowed student IDs) loaded at
startup. add_participant() raises StudentIdNotInRoster when the gate is
on and the supplied id is not present; route returns 403 with a clear
message and logs a roster_reject audit event. Names are NOT checked
against the roster: the join form asks for a current name as a soft
deterrent, but the only hard check is the id.
Includes a deploy/build_roster.py helper that turns class_register
attendance.xlsx into roster.json. Bootstrap env file now exports
QUIZ_ROSTER_PATH; missing file disables the gate (legacy behaviour).
Also drops the user-facing "The cookie is per-device." line from the
join card — students don't need to know the implementation; replaced
with "Enter your registered student ID and your current full name."
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