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>

BUGS.md

@@ -40,18 +40,6 @@ remains the last prerequisite for running unattended.
 
 ## Medium — correctness and robustness
 
-### B-66 — nothing stops rounds from opening with no `fee_address` configured
-
-`app/rounds/config.py:12-17`, `app/rounds/scheduler.py:330-335`.
-
-A fresh instance starts with `fee_address = ""`. Rounds open, bets are accepted and
-confirm, and only then does the payout refuse to build — leaving the round in
-`paying_out`, retrying every 60 s, with the audit log as the only signal.
-
-Fix: refuse to open a round while `fee_address` is unset (and surface it on
-`/admin` and as a maintenance-style banner), so the failure happens before anyone's
-money is committed.
-
 ### B-67 — `/qr/{address}` is unauthenticated, synchronous and only shape-validated
 
 `app/api/routes/qr.py:11-21`.