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_route.py

@@ -94,3 +94,70 @@ async def test_user_played_true_only_for_participants(client):
     resp = await ac.get("/rounds/current")  # no auth at all — logged-out chain-only view
     assert resp.status_code == 200
     assert resp.json()["user_played"] is False
+
+
+async def test_jackpot_comes_from_the_participants_actual_bets(client):
+    """B-11: the jackpot was participant_count * the *current* bet_amount_sats, which
+    overstated it (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."""
+    from sqlalchemy import select
+
+    from app.db.models import Round, RoundConfig, RoundParticipant
+
+    ac, session_factory = client
+
+    async with session_factory() as session:
+        session.add(RoundConfig(fee_address="", bet_amount_sats=1_000_000_000))
+        session.add(Round(id=50, status="open"))
+        await session.flush()
+        # Two bets that actually paid 999_800_000 each (fee deducted), not 1_000_000_000.
+        session.add(
+            RoundParticipant(round_id=50, user_id=1, bet_amount_sats=999_800_000, bet_txid="a", status="confirmed")
+        )
+        session.add(
+            RoundParticipant(round_id=50, user_id=2, bet_amount_sats=999_800_000, bet_txid="b", status="confirmed")
+        )
+        await session.commit()
+
+    body = (await ac.get("/rounds/current")).json()
+    assert body["participant_count"] == 2
+    assert body["jackpot_sats"] == (999_800_000 * 2) * 70 // 100
+
+    # Changing the configured bet amount must not move a running round's jackpot.
+    async with session_factory() as session:
+        config = (await session.scalars(select(RoundConfig))).one()
+        config.bet_amount_sats = 5_000_000_000
+        await session.commit()
+
+    body = (await ac.get("/rounds/current")).json()
+    assert body["jackpot_sats"] == (999_800_000 * 2) * 70 // 100
+
+
+async def test_unhandled_errors_use_the_structured_detail_shape(client):
+    """B-24: the catch-all handler answered with a bare-string `detail`, while
+    app/api/errors.py documents detail as {"code", "message", "params"}. Clients then
+    had to special-case exactly the responses they understand least."""
+    from fastapi import FastAPI
+    from httpx import ASGITransport, AsyncClient
+
+    from app.main import log_unhandled_exception
+
+    app = FastAPI()
+    app.add_exception_handler(Exception, log_unhandled_exception)
+
+    @app.get("/boom")
+    async def boom():
+        raise RuntimeError("secret internal detail")
+
+    transport = ASGITransport(app=app, raise_app_exceptions=False)
+    async with AsyncClient(transport=transport, base_url="http://test") as ac:
+        resp = await ac.get("/boom")
+
+    assert resp.status_code == 500
+    detail = resp.json()["detail"]
+    assert detail["code"] == "internal_error"
+    assert detail["message"] == "internal server error"
+    assert detail["params"] == {}
+    # The exception text belongs in logs/app.log, never in the response body.
+    assert "secret internal detail" not in resp.text