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/services/userService.js

@@ -67,7 +67,7 @@ class UserService {
             const result = await db.runAsync(query, values);
             await db.runAsync('COMMIT');
 
-            return result.lastID;
+            return result.lastInsertRowid;
         } catch (error) {
             await db.runAsync('ROLLBACK');
             logger.error({ err: error }, 'Error creating user');