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/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