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>

app/rounds/service.py

@@ -1,6 +1,9 @@
+from datetime import datetime, timedelta, timezone
+
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.config import settings
 from app.db.models import Round
 
 _ACTIVE_STATUSES = ("open", "closing", "drawing", "paying_out")
@@ -13,14 +16,23 @@ async def get_active_round(session: AsyncSession) -> Round | None:
     return await session.scalar(select(Round).where(Round.status.in_(_ACTIVE_STATUSES)).order_by(Round.id.desc()))
 
 
-async def open_new_round_if_needed(session: AsyncSession) -> Round:
-    """Returns the active round if one exists (whatever its status), otherwise
-    opens a fresh one. Callers that need to attach a bet must additionally check
-    the returned round's status == "open" — a round in closing/drawing/paying_out
-    isn't accepting new bets, but a new round can't open until it's done."""
+async def open_new_round_if_needed(session: AsyncSession) -> Round | None:
+    """Returns the active round if one exists (whatever its status). Otherwise
+    opens a fresh one, unless the last closed round's cooldown (ROUND_COOLDOWN_SECONDS)
+    hasn't elapsed yet — in which case returns None. Callers that need to attach a
+    bet must additionally check the returned round's status == "open" — a round in
+    closing/drawing/paying_out isn't accepting new bets, but a new round can't open
+    until it's done."""
     active = await get_active_round(session)
     if active is not None:
         return active
+
+    last_closed = await session.scalar(select(Round).where(Round.status == "closed").order_by(Round.id.desc()))
+    if last_closed is not None and last_closed.closed_at is not None:
+        closed_at = last_closed.closed_at.replace(tzinfo=timezone.utc)
+        if datetime.now(timezone.utc) < closed_at + timedelta(seconds=settings.round_cooldown_seconds):
+            return None
+
     round_ = Round(status="open")
     session.add(round_)
     await session.flush()