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(),

app/rounds/scheduler.py

@@ -14,7 +14,7 @@ from app.electrum.scripthash import address_to_scripthash
 from app.rounds.config import get_round_config
 from app.rounds.draw import draw_winner, header_hex_to_block_hash
 from app.rounds.events import broadcaster
-from app.rounds.service import open_new_round_if_needed
+from app.rounds.service import open_new_round_if_needed, winner_share
 from app.wallet.hd import derive_pool_key
 from app.wallet.plm_network import PLM_MAINNET
 from app.wallet.psbt_builder import InsufficientFundsError, Utxo, build_payout_transaction
@@ -338,8 +338,8 @@ class RoundScheduler:
             await self._log_payout_failure(round_id, winner_user_id, "winner user not found")
             return
 
-        winner_share = pool_amount_sats * 70 // 100
-        commission_share = pool_amount_sats - winner_share  # remainder from rounding goes to fees
+        winner_sats = winner_share(pool_amount_sats)
+        commission_share = pool_amount_sats - winner_sats  # remainder from rounding goes to fees
 
         # --- Phase 2: build (network read only, no DB write yet) -----------------
         try:
@@ -358,7 +358,7 @@ class RoundScheduler:
                 from_script=pool_script_obj,
                 utxos=utxos,
                 winner_address=winner_address,
-                winner_share_sats=winner_share,
+                winner_share_sats=winner_sats,
                 fee_address=fee_address,
                 commission_sats=commission_share,
                 change_address=pool_address,

app/rounds/service.py

@@ -18,6 +18,14 @@ _ACTIVE_STATUSES = ("open", "closing", "drawing", "paying_out")
 # broken in a way we don't anticipate.
 _OPEN_ROUND_ATTEMPTS = 3
 
+# 70% winner / 30% fees. Hardcoded by design (see CLAUDE.md) — changing the split
+# is a code change, not an admin-editable setting. Single source of truth so the
+# advertised jackpot (rounds.py) and the actual payout (scheduler.py) can't diverge.
+
+
+def winner_share(pool_amount_sats: int) -> int:
+    return pool_amount_sats * 70 // 100
+
 
 async def get_active_round(session: AsyncSession) -> Round | None:
     """The round currently in progress (in any non-closed state), if any. Rounds