Add admin panel for merchant management

- POST/GET/DELETE /api/admin/users endpoints
- Admin UI at /admin with login, user list, create/delete
- Auth via ADMIN_API_KEY env var (separate from tenant auth)
- Tenants remain scoped to their own store only

.env.example

@@ -30,6 +30,9 @@ MONERO_WALLET_RPC_WALLET_DIR=/wallets
 # - Local: run the bundled `monerod` service and set MONERO_DAEMON_URL=http://monerod:18081
 MONERO_DAEMON_URL=http://xmr-node.cakewallet.com:18081
 
+# Admin panel (generate with: openssl rand -base64 32 | tr -d '=' | head -c 32)
+ADMIN_API_KEY=
+
 # Registration (set to false to require manual merchant onboarding)
 OPEN_REGISTRATION=true