Add a cooldown between rounds (ROUND_COOLDOWN_SECONDS)

open_new_round_if_needed now withholds opening the next round until
ROUND_COOLDOWN_SECONDS (default 30) have passed since the previous
round's closed_at, returning None in that window instead of a Round.
Without this, the next round opened within one scheduler tick (~5s) of
the previous payout confirming — not enough time for a player to
notice the round they were in actually resolved.

Callers updated: the scheduler treats None as "nothing to do this
tick", and place_bet raises a "try again shortly" BetError instead of
crashing on a None round.

Not in the original flowchart — a deliberate UX addition on top of it,
documented as such in CLAUDE.md.

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

tests/unit/test_rounds_service.py

@@ -1,6 +1,9 @@
+from datetime import datetime, timedelta, timezone
+
 import pytest
 from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
 
+from app.config import settings
 from app.db.base import Base
 from app.db.models import Round
 from app.rounds.service import get_active_round, open_new_round_if_needed
@@ -56,3 +59,24 @@ async def test_opens_new_round_after_previous_is_closed(session_factory):
     async with session_factory() as session:
         round_ = await open_new_round_if_needed(session)
         assert round_.status == "open"
+
+
+async def test_withholds_new_round_during_cooldown(session_factory):
+    async with session_factory() as session:
+        session.add(Round(status="closed", closed_at=datetime.now(timezone.utc)))
+        await session.commit()
+
+    async with session_factory() as session:
+        round_ = await open_new_round_if_needed(session)
+        assert round_ is None
+
+
+async def test_opens_new_round_once_cooldown_elapses(session_factory):
+    stale_close = datetime.now(timezone.utc) - timedelta(seconds=settings.round_cooldown_seconds + 1)
+    async with session_factory() as session:
+        session.add(Round(status="closed", closed_at=stale_close))
+        await session.commit()
+
+    async with session_factory() as session:
+        round_ = await open_new_round_if_needed(session)
+        assert round_.status == "open"