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/bets/service.py

@@ -21,6 +21,8 @@ class BetError(Exception):
 
 async def place_bet(session: AsyncSession, client: ElectrumClient, user: User) -> RoundParticipant:
     round_ = await open_new_round_if_needed(session)
+    if round_ is None:
+        raise BetError("no round open right now, please try again shortly")
     if round_.status != "open":
         raise BetError("the current round is closing, please try again shortly")