Implement full MVP: auth, HD wallet, Electrum client, deposits, bets, round/draw engine, payout, withdrawals, RBF, admin+audit

All 10 build-order stages complete and unit-tested (49 tests). Verified live on
mainnet: registration/address derivation, deposit crediting, a real 10 PLM bet
(broadcast + confirmed + change credited). A full round close->draw->payout
cycle was triggered live and was in progress at commit time. Withdrawal and RBF
bump are unit-tested but not yet exercised against a live broadcast. Known gaps
(scheduler doesn't resume mid-flight rounds after restart, payout has no retry,
no deployment setup, etc.) are documented in CLAUDE.md.

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

app/wallet/balance.py

@@ -0,0 +1,16 @@
+from sqlalchemy import func, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.db.models import User, UtxoEvent
+
+
+async def recompute_balance(session: AsyncSession, user_id: int) -> int:
+    """Source of truth: sum of this user's confirmed, unspent UTXOs. Updates and
+    returns the read-cache column (User.cached_balance_sats). Must be called
+    within the same transaction as whatever inserted/updated utxo_events rows."""
+    balance = await session.scalar(
+        select(func.sum(UtxoEvent.amount_sats)).where(UtxoEvent.user_id == user_id, UtxoEvent.spent_txid.is_(None))
+    )
+    user = await session.get(User, user_id)
+    user.cached_balance_sats = balance or 0
+    return user.cached_balance_sats