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/static/admin.html

@@ -62,6 +62,13 @@
       </div>
 
       <div class="card">
+        <!-- B-66: no fee_address means no round can open at all (the payout pays the
+             30% commission to it, so it cannot even be built). Shown here because
+             this is the one screen that can fix it. -->
+        <div class="warning-banner hidden" id="admin-fee-address-warning">
+          ⚠️ Nessun fee address configurato: finché resta vuoto <strong>non si aprirà nessun round</strong>
+          (il payout non sarebbe costruibile). Impostalo qui sotto e salva.
+        </div>
         <div class="grid-2">
           <div>
             <label for="admin-fee-address">Fee address (dove finisce il 30% di ogni round)</label>