Snapshot a round's timing when it opens (B-61)

round_duration_seconds was read live on every scheduler tick and every bet
check, with the deadline computed as opened_at + duration. Lowering it from 600
to 60 while a round was 300s in closed that round instantly; raising it moved
the closes_at clients were already counting down to. round_cooldown_seconds had
the same property for the gap after a close. B-11 fixed this class of problem
for the advertised jackpot; the timing fields were left live.

Round now carries duration_seconds and cooldown_seconds, set from the config
when it opens. round_deadline() is the single place the deadline is computed —
the scheduler, place_bet's two checks and /rounds/current's closes_at all go
through it — and the cooldown is read off the round that just closed, so the gap
a round announced is the gap that's honoured. The config row becomes what the
*next* round opens with.

The migration backfills from the live config rather than leaving the column
defaults: an instance running 300s rounds would otherwise see the round
currently in progress jump to 600s the moment this lands, which is precisely the
retroactive change being fixed. Verified against a scratch DB with a non-default
config.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/rounds/scheduler.py

@@ -14,7 +14,7 @@ from app.electrum.scripthash import address_to_scripthash
 from app.rounds.config import get_round_config
 from app.rounds.draw import draw_winner, header_hex_to_block_hash
 from app.rounds.events import broadcaster
-from app.rounds.service import open_new_round_if_needed, winner_share
+from app.rounds.service import open_new_round_if_needed, round_deadline, winner_share
 from app.wallet.hd import derive_pool_key
 from app.wallet.plm_network import PLM_MAINNET
 from app.wallet.psbt_builder import InsufficientFundsError, Utxo, build_payout_transaction
@@ -67,8 +67,11 @@ class RoundScheduler:
             await session.commit()
             if round_ is None:
                 return  # still in the cooldown window after the last round closed
-            round_id, status, opened_at = round_.id, round_.status, round_.opened_at
-            round_duration_seconds = (await get_round_config(session)).round_duration_seconds
+            round_id, status = round_.id, round_.status
+            # B-61: this round's own snapshotted deadline, not one recomputed from
+            # whatever the config says now — an operator lowering the duration
+            # mid-round used to close the round on the spot.
+            deadline = round_deadline(round_)
 
         if status == "paying_out":
             # B-26: _trigger_payout used to run exactly once, from _close_and_draw —
@@ -83,8 +86,7 @@ class RoundScheduler:
             return  # "drawing" — progress happens inside the in-flight _close_and_draw call
 
         if status == "open":
-            opened_at = opened_at.replace(tzinfo=timezone.utc)
-            if datetime.now(timezone.utc) < opened_at + timedelta(seconds=round_duration_seconds):
+            if datetime.now(timezone.utc) < deadline:
                 return
 
             async with self._session_factory() as session: