Deduplicate the 70/30 prize split formula

pool_amount_sats * 70 // 100 was hardcoded identically in both
rounds/scheduler.py (the actual payout) and api/routes/rounds.py (the
advertised jackpot). They happened to agree, but nothing enforced it —
changing one without the other would have made GET /rounds/current's
jackpot silently diverge from the real payout. Extract winner_share()
into rounds/service.py as the single source of truth.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/api/routes/rounds.py

@@ -15,7 +15,7 @@ from app.db.models import RoundParticipant, User
 from app.db.session import get_session
 from app.rounds.config import get_round_config
 from app.rounds.events import EVICTED, RoundEventCapacityError, broadcaster
-from app.rounds.service import get_active_round
+from app.rounds.service import get_active_round, winner_share
 
 router = APIRouter(prefix="/rounds", tags=["rounds"])
 
@@ -165,7 +165,7 @@ async def current_round(
     # upper bound by the payout tx's own fee, which is deducted from the winner's
     # share and isn't knowable until the payout is built — a few hundred sat on a
     # 1 sat/vB payout, i.e. invisible at PLM amounts, but it is not exact.
-    jackpot_sats = pool_amount_sats * 70 // 100
+    jackpot_sats = winner_share(pool_amount_sats)
 
     return CurrentRoundResponse(
         server_time=datetime.now(timezone.utc).isoformat(),