Add admin config and audit log

Bearer-token-gated admin endpoints to read/update the DB-backed
operational config (fee_address, bet_amount_sats) without a redeploy,
plus a lightweight audit log writer for round/payout/config events.

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,
+        )
+    )