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>

migrations/versions/283844a44b4a_snapshot_round_timing_onto_the_round_.py

@@ -0,0 +1,42 @@
+"""snapshot round timing onto the round row (B-61)
+
+Revision ID: 283844a44b4a
+Revises: c1d4a97b5e10
+Create Date: 2026-08-03 23:05:04.996492
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '283844a44b4a'
+down_revision: Union[str, Sequence[str], None] = 'c1d4a97b5e10'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.add_column('rounds', sa.Column('duration_seconds', sa.Integer(), server_default='600', nullable=False))
+    op.add_column('rounds', sa.Column('cooldown_seconds', sa.Integer(), server_default='30', nullable=False))
+
+    # Backfill from the live config rather than leaving the column defaults: an
+    # instance running with, say, a 300s round would otherwise see every existing
+    # row — including the round currently in progress — jump to 600s the moment
+    # this migration lands, which is exactly the retroactive change B-61 is about.
+    op.execute(
+        "UPDATE rounds SET "
+        "duration_seconds = coalesce((SELECT round_duration_seconds FROM round_config LIMIT 1), 600), "
+        "cooldown_seconds = coalesce((SELECT round_cooldown_seconds FROM round_config LIMIT 1), 30)"
+    )
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_column('rounds', 'cooldown_seconds')
+    op.drop_column('rounds', 'duration_seconds')
+    # ### end Alembic commands ###