feat(roster): gate joins on registered student-ID list

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."

deploy/bootstrap.sh

@@ -105,6 +105,7 @@ if [ ! -f "$ENV_FILE" ]; then
         cat > "$ENV_FILE" <<EOF
 QUIZ_DB_PATH=$APP_DIR/quiz.db
 QUIZ_POOL_PATH=$APP_DIR/pool.json
+QUIZ_ROSTER_PATH=$APP_DIR/roster.json
 QUIZ_SECRET_KEY=$QUIZ_SECRET_KEY
 QUIZ_ADMIN_PASSWORD=$QUIZ_ADMIN_PASSWORD
 QUIZ_HOST=127.0.0.1