Fix the round-open race, the advertised jackpot, and payout error handling

Round opening (B-09). open_new_round_if_needed now handles the IntegrityError
from ix_rounds_single_active (previous commit) by rolling back and using the
winner's round. Deviation from the plan in BUGS.md, which proposed making the
scheduler the only writer: that would mean the first bet after a cooldown
couldn't open a round, so both callers stay and a bounded retry was added
instead — a conflict where nothing is active yet just means the winner hadn't
committed, and a bet must not fail on that timing. get_active_round also logs
loudly if it ever sees more than one active round rather than silently picking
the newest.

The jackpot (B-11). It was participant_count * the *current* bet_amount_sats,
which overstated the pool (each stored bet is already net of that bet's network
fee) and silently rewrote the advertised jackpot of a round in progress whenever
an operator edited the bet amount. It now sums the participants' stored
bet_amount_sats. The remaining imprecision — the payout tx's own fee, deducted
from the winner's share and unknowable until the payout is built — is documented
in the code rather than promised away, since the comment there claimed exactness.

Payout (B-05, B-18). _trigger_payout is split into read / build+broadcast /
persist, so no DB session is held across a network call (on SQLite that meant
holding the write lock for two unbounded round-trips). That restructuring is also
what makes the error handling placeable: it now catches Exception around the
chain work and writes a payout_failed audit entry, where a malformed fee_address
used to raise EmbitError all the way to the scheduler's catch-all, leaving the
round stuck in paying_out with nothing recorded about why. Automatic payout retry
remains an open gap.

The scheduler also counts "building" participants as in-flight when deciding
whether a round may close, matching the two-phase bet write.

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

tests/unit/test_rounds_service.py

@@ -1,6 +1,7 @@
 from datetime import datetime, timedelta, timezone
 
 import pytest
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 
 from app.db.base import Base
@@ -105,3 +106,68 @@ async def test_pause_does_not_interrupt_a_round_in_progress(session_factory):
         returned = await open_new_round_if_needed(session)
         assert returned is not None
         assert returned.status == "drawing"
+
+
+async def test_losing_the_open_race_reuses_the_winning_round(session_factory, monkeypatch):
+    """B-09: open_new_round_if_needed was a read-then-insert with no lock, called from
+    both the scheduler and every place_bet, so two callers could both see "no active
+    round" and insert one — and a second stuck "open" row blocks every future round,
+    since get_active_round matches on status.
+
+    The race is forced deterministically: the round already exists and is committed,
+    but this caller's first look is made to miss it (exactly what the loser of the
+    race sees). The insert then hits ix_rounds_single_active, and the caller must
+    recover by using the winner's round instead of raising at its caller — a bet must
+    not fail because a scheduler tick beat it by a millisecond.
+    """
+    from app.rounds import service as service_module
+
+    async with session_factory() as session:
+        session.add(Round(status="open"))
+        await session.commit()
+
+    real_get_active_round = service_module.get_active_round
+    calls = {"n": 0}
+
+    async def blind_first_look(session):
+        calls["n"] += 1
+        if calls["n"] == 1:
+            return None  # what the loser of the race sees
+        return await real_get_active_round(session)
+
+    monkeypatch.setattr(service_module, "get_active_round", blind_first_look)
+
+    async with session_factory() as session:
+        round_ = await service_module.open_new_round_if_needed(session)
+        await session.commit()
+
+    assert round_ is not None  # recovered, didn't raise
+    async with session_factory() as session:
+        rounds = (await session.scalars(select(Round))).all()
+        assert len(rounds) == 1, f"expected one round, got {[(r.id, r.status) for r in rounds]}"
+        assert round_.id == rounds[0].id  # the winner's round, not a second one
+
+
+async def test_the_database_refuses_a_second_active_round(session_factory):
+    """The guarantee itself, independent of the application code path."""
+    from sqlalchemy.exc import IntegrityError
+
+    async with session_factory() as session:
+        session.add(Round(status="open"))
+        await session.commit()
+
+    async with session_factory() as session:
+        session.add(Round(status="drawing"))
+        with pytest.raises(IntegrityError):
+            await session.commit()
+
+
+async def test_closed_rounds_can_coexist_with_an_active_one(session_factory):
+    async with session_factory() as session:
+        session.add(Round(status="closed"))
+        session.add(Round(status="closed"))
+        session.add(Round(status="open"))
+        await session.commit()
+
+    async with session_factory() as session:
+        assert len((await session.scalars(select(Round))).all()) == 3