feat: web admin panel + better-sqlite3 migration + Docker fixes

- Added Express.js admin panel on port 3001 (ADMIN_PORT env)
  - Dashboard: stats (users, products, purchases, revenue)
  - Users: list, details, ban/unban toggle
  - Products: CRUD by category
  - Wallets: list with balances
  - Purchases: history with filters
  - Audit log: view audit trail
  - Auth: token-based login with ADMIN_SECRET env var
- Migrated sqlite3 → better-sqlite3
  - database.js: async adapter (runAsync/allAsync/getAsync)
  - purchaseService.js: lastID → lastInsertRowid
  - userService.js: lastID → lastInsertRowid
  - Removed sqlite3 from package.json
- Fixed: dotenv/config import added to index.js
- Fixed: ENCRYPTION_KEY validation (32+ char hex)
- Fixed: Dockerfile multi-stage build (no python needed)
- Fixed: Docker DNS (network: host in build)
- Fixed: docker-compose port 3001, healthcheck on 3001
- Added express, cookie-parser, pino-pretty, better-sqlite3 deps

src/admin/server.js

@@ -0,0 +1,46 @@
+import express from 'express';
+import cookieParser from 'cookie-parser';
+import { fileURLToPath } from 'url';
+import { dirname, join } from 'path';
+import logger from '../utils/logger.js';
+import { requireAuth, handleLogin, handleLogout, renderLogin } from './auth.js';
+import dashboardRouter from './routes/dashboard.js';
+import usersRouter from './routes/users.js';
+import productsRouter from './routes/products.js';
+import walletsRouter from './routes/wallets.js';
+import purchasesRouter from './routes/purchases.js';
+import auditRouter from './routes/audit.js';
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const app = express();
+
+app.use(cookieParser());
+app.use(express.urlencoded({ extended: true }));
+app.use('/admin/style.css', express.static(join(__dirname, 'public', 'style.css')));
+
+app.get('/health', (req, res) => {
+  res.json({ status: 'ok', uptime: process.uptime() });
+});
+
+app.get('/login', (req, res) => {
+  res.send(renderLogin());
+});
+
+app.post('/login', handleLogin);
+app.get('/logout', handleLogout);
+
+app.use(requireAuth);
+
+app.use('/', dashboardRouter);
+app.use('/users', usersRouter);
+app.use('/products', productsRouter);
+app.use('/wallets', walletsRouter);
+app.use('/purchases', purchasesRouter);
+app.use('/audit', auditRouter);
+
+export function startAdminPanel() {
+  const port = parseInt(process.env.ADMIN_PORT || '3001', 10);
+  app.listen(port, () => {
+    logger.info({ port }, 'Admin panel started');
+  });
+}