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/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,