Add admin endpoints to list users and export a user's private key

GET /admin/users lists id/username/address/balance_sats/created_at.
GET /admin/users/{id}/privkey derives and returns that user's raw WIF
private key, for manual intervention (e.g. sweeping funds back if
something's stuck) — this is already a custodial system, the server
holds the master key everything is derived from, so this doesn't grant
a new capability, just exposes an existing one through the API. Every
access is audit-logged (admin_privkey_accessed).

Also fixes a pre-existing test-isolation bug in test_admin.py's client
fixture: app.db.session.get_session had `from app.db.base import
AsyncSessionLocal`, a one-time reference copy at first import — later
tests reassigning db_base.AsyncSessionLocal never reached it, so any
test mixing direct DB writes with router calls silently read/wrote
against a stale, possibly-disposed engine from whichever test ran
first. Fixed by also rebinding app.db.session.AsyncSessionLocal in the
fixture on every run.

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

app/api/routes/admin.py

@@ -1,10 +1,14 @@
 from fastapi import APIRouter, Depends, Header, HTTPException, status
 from pydantic import BaseModel
+from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.audit.log import write_audit_log
 from app.config import settings
+from app.db.models import User
 from app.db.session import get_session
 from app.rounds.config import get_round_config
+from app.wallet.hd import derive_user_wif
 
 router = APIRouter(prefix="/admin", tags=["admin"])
 
@@ -42,3 +46,48 @@ async def update_config(
         config.bet_amount_sats = body.bet_amount_sats
     await session.commit()
     return RoundConfigResponse(fee_address=config.fee_address, bet_amount_sats=config.bet_amount_sats)
+
+
+class AdminUserResponse(BaseModel):
+    id: int
+    username: str
+    address: str
+    balance_sats: int
+    created_at: str
+
+
+@router.get("/users", response_model=list[AdminUserResponse], dependencies=[Depends(require_admin)])
+async def list_users(session: AsyncSession = Depends(get_session)) -> list[AdminUserResponse]:
+    users = (await session.scalars(select(User).order_by(User.id))).all()
+    return [
+        AdminUserResponse(
+            id=u.id,
+            username=u.username,
+            address=u.address,
+            balance_sats=u.cached_balance_sats,
+            created_at=u.created_at.isoformat(),
+        )
+        for u in users
+    ]
+
+
+class AdminPrivkeyResponse(BaseModel):
+    address: str
+    wif: str
+
+
+@router.get(
+    "/users/{user_id}/privkey", response_model=AdminPrivkeyResponse, dependencies=[Depends(require_admin)]
+)
+async def user_privkey(user_id: int, session: AsyncSession = Depends(get_session)) -> AdminPrivkeyResponse:
+    """Exports a user's raw private key for manual intervention (e.g. sweeping
+    funds back if something's stuck). Every access is audit-logged since this is
+    the most sensitive data the platform holds."""
+    user = await session.get(User, user_id)
+    if user is None:
+        raise HTTPException(status.HTTP_404_NOT_FOUND, "user not found")
+
+    wif = derive_user_wif(user.derivation_index)
+    await write_audit_log(session, "admin_privkey_accessed", {"user_id": user_id}, user_id=user_id)
+    await session.commit()
+    return AdminPrivkeyResponse(address=user.address, wif=wif)