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/db/models.py

@@ -82,6 +82,16 @@ class Round(Base):
     status: Mapped[str] = mapped_column(String(16), default="open")
     opened_at: Mapped[datetime] = mapped_column(default=utcnow)
     closed_at: Mapped[datetime | None] = mapped_column(default=None)
+    # B-61: the round's own timing, snapshotted from RoundConfig when it opens.
+    # Read live from the config, a mid-round edit applied retroactively: lowering
+    # round_duration_seconds from 600 to 60 while a round was 300s in closed it
+    # instantly, and raising it moved the closes_at every client was already
+    # counting down to. Same class of bug B-11 fixed for the advertised jackpot.
+    # The config row is now what the *next* round opens with; these are what this
+    # round runs by. cooldown_seconds is read off the round that just closed, so
+    # the gap it announced is the gap that's honoured.
+    duration_seconds: Mapped[int] = mapped_column(default=600, server_default="600")
+    cooldown_seconds: Mapped[int] = mapped_column(default=30, server_default="30")
     # Set once, when status flips to "drawing" (rounds/scheduler.py:_close_and_draw).
     # Lets both the audit log (B-36's draw_stalled entries) and GET /rounds/current
     # (draw_waiting_since) measure how long a round has been waiting on a block,