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>

app/api/routes/rounds.py

@@ -15,7 +15,7 @@ from app.db.models import RoundParticipant, User
 from app.db.session import get_session
 from app.rounds.config import get_round_config
 from app.rounds.events import EVICTED, RoundEventCapacityError, broadcaster
-from app.rounds.service import get_active_round, round_deadline, winner_share
+from app.rounds.service import get_active_round, round_deadline, rounds_can_open, winner_share
 
 router = APIRouter(prefix="/rounds", tags=["rounds"])
 
@@ -117,6 +117,12 @@ class CurrentRoundResponse(BaseModel):
     draw_waiting_since: str | None = None
     chain_tip_height: int | None = None
     lottery_paused: bool = False
+    # B-66: false while the instance is missing configuration a round cannot run
+    # without (today: fee_address) — no round will open until it's set, so this is
+    # the difference between "wait, the next round is coming" and "nothing is coming
+    # until the operator finishes setting this up". Distinct from lottery_paused,
+    # which is a deliberate operator action rather than an unmet prerequisite.
+    lottery_configured: bool = True
     user_played: bool = False
 
 
@@ -138,6 +144,7 @@ async def current_round(
             draw_animation_seconds=config.draw_animation_seconds,
             chain_tip_height=chain_tip_height,
             lottery_paused=config.paused,
+            lottery_configured=rounds_can_open(config),
         )
 
     # The pool is the sum of what the participants' bets actually paid into the pool
@@ -214,5 +221,6 @@ async def current_round(
         draw_waiting_since=isoformat_utc(round_.drawing_started_at) if round_.status == "drawing" else None,
         chain_tip_height=chain_tip_height,
         lottery_paused=config.paused,
+        lottery_configured=rounds_can_open(config),
         user_played=user_played,
     )