Distinguish round-closing sub-phases and anchor cooldown to payout confirmation

Flip status to "closing" the instant the round timer expires, rather than
leaving it "open" (invisible) while in-flight bets confirm — /rounds/current
now surfaces this wait as its own phase instead of collapsing it into
"drawing". _tick() is updated to keep re-checking pending bets while status
is "closing" instead of short-circuiting on the old "status != open" guard.

Also re-stamp closed_at at actual payout confirmation time (not at the
earlier "closing" transition), since round_cooldown_seconds counts from
closed_at — with a short cooldown (e.g. 20s) and ~2 block-time draw+payout
wait, the old anchor meant the cooldown had already elapsed by the time the
round actually closed, making it a no-op.

Expose draw_block_height/draw_block_hash on GET /rounds/current so clients
can show which block the winner was drawn from.

app/rounds/confirmation.py

@@ -1,3 +1,5 @@
+from datetime import datetime, timezone
+
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
@@ -9,6 +11,12 @@ async def _on_payout_confirmed(session: AsyncSession, pending: PendingTransactio
     round_ = await session.scalar(select(Round).where(Round.payout_txid == pending.current_txid))
     if round_ is not None and round_.status == "paying_out":
         round_.status = "closed"
+        # closed_at is what round_cooldown_seconds counts from (service.py's
+        # open_new_round_if_needed) — re-stamp it here at actual payout
+        # confirmation time rather than leaving it at the earlier "closing"
+        # timestamp, so a short cooldown (e.g. 20s) is a real pause after the
+        # winner's tx confirms, not swallowed by the ~2 block-time draw+payout wait.
+        round_.closed_at = datetime.now(timezone.utc)
         # The winner's own address is already watched by the Electrum listener, so
         # their balance is credited by the normal deposit path once this confirms.