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/scheduler.py

@@ -50,6 +50,8 @@ class RoundScheduler:
         async with self._session_factory() as session:
             round_ = await open_new_round_if_needed(session)
             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
 
         if status != "open":