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/audit/log.py

@@ -0,0 +1,22 @@
+import json
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.db.models import AuditLog
+
+
+async def write_audit_log(
+    session: AsyncSession,
+    event_type: str,
+    payload: dict,
+    user_id: int | None = None,
+    round_id: int | None = None,
+) -> None:
+    session.add(
+        AuditLog(
+            event_type=event_type,
+            payload_json=json.dumps(payload),
+            user_id=user_id,
+            round_id=round_id,
+        )
+    )