Refuse to open a round that could not pay its winner (B-66)

fee_address has no column default, because an operator has to supply their own —
and the payout pays the 30% commission to it, so build_payout_transaction cannot
even be built without one. A fresh instance nonetheless opened rounds happily:
each took bets, confirmed them, and only then discovered it was unpayable,
wedging in "paying_out" and retrying every 60s with money already in the pool.
One manual recovery per round, until somebody noticed.

open_new_round_if_needed now checks rounds_can_open(config) alongside `paused`:
no payout address, no round. Nothing has moved yet at that point, which is the
whole difference. Same scope as pausing — a round already in progress still
closes, draws and pays out, since clearing the address mid-round is exactly the
operator slip that must not strand a live round.

Surfaced rather than silent, in the two places that matter: lottery_configured on
GET /rounds/current, which makes / show a *different* banner from the maintenance
one (telling a player "come back later" would be false — nothing is coming until
setup finishes), and a warning at the top of /admin's Parametri card, the one
screen that can fix it. rounds_can_open is where any future
would-make-a-round-unpayable prerequisite belongs, instead of being discovered at
payout time.

The test churn is the finding restated: 26 tests expected a round to open on an
instance with no payout address. Their fixtures now seed one, so each goes back to
testing what it says — several would otherwise have passed for the wrong reason,
returning None because of the missing address rather than because of the cooldown
or pause under test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/test_withdrawals.py

@@ -5,12 +5,15 @@ from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 from app.bets.service import place_bet
 from app.config import settings
 from app.db.base import Base
-from app.db.models import PendingTransaction, User, UtxoEvent, Withdrawal
+from app.db.models import PendingTransaction, RoundConfig, User, UtxoEvent, Withdrawal
 from app.rounds.events import broadcaster
 from app.wallet.hd import derive_user_address
 from app.withdrawals.service import WithdrawalError, request_withdrawal
 
 
+_FEE_ADDRESS = "plm1q5x25wd6q463mfhckjraaedgjg0lyu73qfcj43n"
+
+
 class FakeElectrumClient:
     def __init__(self):
         self.broadcasted: list[str] = []
@@ -40,6 +43,13 @@ async def session_factory(tmp_path, monkeypatch):
     engine = create_async_engine("sqlite+aiosqlite:///:memory:")
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
+
+    # B-66: a round only opens on an instance that could actually pay a winner, so
+    # every test that expects one needs a fee address configured — the column has no
+    # default on purpose (an operator must set their own).
+    async with async_sessionmaker(engine, expire_on_commit=False)() as session:
+        session.add(RoundConfig(fee_address=_FEE_ADDRESS))
+        await session.commit()
     yield async_sessionmaker(engine, expire_on_commit=False)
     await engine.dispose()
     hd._account_key = None