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/rounds/confirmation.py

@@ -0,0 +1,16 @@
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.db.models import PendingTransaction, Round
+from app.tx.confirmation import register_handler
+
+
+async def _on_payout_confirmed(session: AsyncSession, pending: PendingTransaction) -> None:
+    round_ = await session.scalar(select(Round).where(Round.payout_txid == pending.current_txid))
+    if round_ is not None and round_.status == "paying_out":
+        round_.status = "closed"
+        # The winner's own address is already watched by the Electrum listener, so
+        # their balance is credited by the normal deposit path once this confirms.
+
+
+register_handler("payout", _on_payout_confirmed)