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/wallet/hd.py

@@ -2,6 +2,7 @@ import os
 
 from embit import script
 from embit.bip32 import HDKey
+from embit.ec import PrivateKey
 
 from app.config import settings
 from app.wallet.keystore import decrypt_xprv, encrypt_xprv
@@ -39,6 +40,16 @@ def derive_user_address(derivation_index: int) -> str:
     return script.p2wpkh(pub).address(network=PLM_MAINNET)
 
 
+def derive_user_wif(derivation_index: int) -> str:
+    """Exports a user's raw private key (WIF) for manual server-side intervention
+    (e.g. sweeping funds back to a user, or out, if something gets stuck). This is
+    a custodial system — the server already holds the master key this is derived
+    from — but callers must still treat the result as a live secret: log access,
+    never persist it, never return it over an unauthenticated channel."""
+    key = derive_user_key(derivation_index)
+    return PrivateKey(key.secret, compressed=True, network=PLM_MAINNET).wif(network=PLM_MAINNET)
+
+
 def derive_pool_key() -> HDKey:
     """The "indirizzo padre" from the flowchart: all bets are sent here, and
     payouts are signed with this key. Reserved on branch 1 of the account (branch 0