Restore accidentally removed files, a few additional import/calculation fixes

inventory-server/src/utils/db.js

@@ -0,0 +1,21 @@
+const { Pool } = require('pg');
+
+let pool;
+
+function initPool(config) {
+  pool = new Pool(config);
+  return pool;
+}
+
+async function getConnection() {
+  if (!pool) {
+    throw new Error('Database pool not initialized');
+  }
+  return pool.connect();
+}
+
+module.exports = {
+  initPool,
+  getConnection,
+  getPool: () => pool
+};