fix: hide score on submit + total denominator + projector chart cleanup

Three small UX/fairness tweaks from manual live testing:

1. Post-submit "wait for reveal" screen: show only the response time, no
   score. The +score reveal leaked correctness — any positive number =
   correct, zero = wrong — short-circuiting the "stop and think" beat
   the reveal pause was supposed to enforce. Time stays as the
   engagement signal; score now waits for the instructor reveal.

2. Final-screen "Correct X / Y" denominator is now total_questions
   instead of questions_answered. Missed questions are scored zero, so
   they belong in the denominator visibly. Server adds total_questions
   to the session_ended payload.

3. Projector score-distribution: drop the in-chart count labels (they
   collided with each other and with the median tag at small N), restore
   the previously-computed-but-not-rendered x-axis tick labels at the
   bottom. Stats line at the foot keeps n / mean / max.

Also: short-circuit the per-submit instructor + presence broadcasts
when no instructor / projector is connected (no listener, no DB work).
The 50-student load test was tight on margin against its 2 s
time_limit; with the new presence_message / live_histogram_message DB
queries firing on every submit, the margin disappeared on busy boxes.
Conftest fixture also bumped to 8 s per question for the same reason —
gives breathing room for sequential WS submits in the load test.

71/71 pytest green.

tests/test_pool.py

@@ -7,7 +7,7 @@ def test_pool_validation_accepts_well_formed_pool(sample_pool):
     pool = parse_pool_json(sample_pool)
     assert pool["title"] == "Sample Quiz"
     assert pool["score_fn"] == "linear_decay"
-    assert question_time_limit(pool, 0) == 2
+    assert question_time_limit(pool, 0) == 8
     assert get_question(pool, 0)["correct"] == "B"
     public = public_question_payload(pool, 0)
     assert "correct" not in public