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/api/routes/rounds.py

@@ -25,6 +25,8 @@ class CurrentRoundResponse(BaseModel):
     draw_animation_seconds: int
     winner_user_id: int | None = None
     winner_amount_sats: int | None = None
+    draw_block_height: int | None = None
+    draw_block_hash: str | None = None
     chain_tip_height: int | None = None
     lottery_paused: bool = False
 
@@ -70,6 +72,8 @@ async def current_round(request: Request, session: AsyncSession = Depends(get_se
         draw_animation_seconds=config.draw_animation_seconds,
         winner_user_id=round_.winner_user_id,
         winner_amount_sats=round_.winner_amount_sats,
+        draw_block_height=round_.draw_block_height,
+        draw_block_hash=round_.draw_block_hash,
         chain_tip_height=chain_tip_height,
         lottery_paused=config.paused,
     )