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>

CLAUDE.md

@@ -62,6 +62,7 @@ Known risk: `docker-compose.yml` sets `restart: unless-stopped` on `app`, so a c
 - **Secrets**: master xprv encrypted at rest with a symmetric scheme (AES-GCM/Fernet); the encryption key itself lives in an env var, never in the DB or in git.
 - **Operational config** (fee/commission address, RBF fee-bump wallet, etc.): stored in a DB config table, not env vars — must be editable without a redeploy.
 - **Round duration**: configurable via env var, default 10 minutes (not hardcoded).
+- **Round cooldown**: `ROUND_COOLDOWN_SECONDS` (default 30s) — gap after a round closes before the next one opens, so players have time to see the outcome. Not in the original flowchart; added afterwards as an explicit design decision.
 
 ## PLM network parameters