Make a stalled draw wait observable (B-36)

_wait_for_next_block had no timeout, no log, and no audit entry: a
connection that stopped advancing the tip left a round silently frozen
in "drawing" with nothing in /admin to explain why. Log progress
periodically, write a draw_stalled audit entry past a threshold (a few
block-time multiples), and surface the wait via a new Round.drawing_started_at
column, exposed as draw_waiting_since in GET /rounds/current.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/unit/test_scheduler.py

@@ -381,7 +381,9 @@ async def test_wait_for_next_block_accepts_an_immediately_corroborated_block(ses
     listener = CorroboratingListener(responses={101: True})
     scheduler = RoundScheduler(session_factory, listener)
 
-    height, block_hash = await scheduler._wait_for_next_block(round_id=1, tip_at_close=100)
+    height, block_hash = await scheduler._wait_for_next_block(
+        round_id=1, tip_at_close=100, waiting_since=datetime.now(timezone.utc)
+    )
 
     assert height == 101
     assert listener.corroboration_calls == [101]
@@ -395,7 +397,9 @@ async def test_wait_for_next_block_retries_after_a_failed_corroboration(session_
     )
     scheduler = RoundScheduler(session_factory, listener)
 
-    height, block_hash = await scheduler._wait_for_next_block(round_id=1, tip_at_close=100)
+    height, block_hash = await scheduler._wait_for_next_block(
+        round_id=1, tip_at_close=100, waiting_since=datetime.now(timezone.utc)
+    )
 
     assert height == 102
     assert listener.corroboration_calls == [101, 102]
@@ -403,3 +407,53 @@ async def test_wait_for_next_block_retries_after_a_failed_corroboration(session_
     async with session_factory() as session:
         events = [e.event_type for e in (await session.scalars(select(AuditLog))).all()]
         assert events == ["draw_header_corroboration_failed"]
+
+
+# --- B-36: a stalled draw must be visible, not a silent frozen wait --------------
+
+
+class StallingListener:
+    """A tip that never advances until the test decides it should — used to drive
+    _wait_for_next_block's stall-detection past _DRAW_STALL_THRESHOLD_SECONDS
+    without a real 6-minute wait."""
+
+    def __init__(self):
+        self.tip_height = 100
+        self.tip_header_hex = None
+
+    async def corroborate_header(self, height: int, expected_hash: str) -> bool:
+        return True
+
+
+async def test_wait_for_next_block_logs_a_stall_audit_entry_past_the_threshold(session_factory, monkeypatch):
+    import app.rounds.scheduler as scheduler_module
+
+    listener = StallingListener()
+    scheduler = RoundScheduler(session_factory, listener)
+    start = datetime.now(timezone.utc)
+
+    class _FakeClock:
+        now = start
+
+    def fake_now(tz=None):
+        return _FakeClock.now
+
+    async def fake_sleep(seconds: float) -> None:
+        _FakeClock.now += timedelta(seconds=seconds)
+        # Past the stall threshold, but before it would repeat: unblock the wait
+        # by making a (corroborated) block appear, so the test terminates.
+        if _FakeClock.now >= start + timedelta(seconds=scheduler_module._DRAW_STALL_THRESHOLD_SECONDS + 30):
+            listener.tip_height = 101
+            listener.tip_header_hex = "aa"
+
+    monkeypatch.setattr(scheduler_module, "datetime", type("_D", (), {"now": staticmethod(fake_now)}))
+    monkeypatch.setattr(scheduler_module.asyncio, "sleep", fake_sleep)
+
+    height, block_hash = await scheduler._wait_for_next_block(round_id=1, tip_at_close=100, waiting_since=start)
+
+    assert height == 101
+
+    async with session_factory() as session:
+        entries = (await session.scalars(select(AuditLog).where(AuditLog.event_type == "draw_stalled"))).all()
+        assert len(entries) == 1
+        assert entries[0].round_id == 1