feat: add environment configuration and required variables

.env.example

@@ -0,0 +1,4 @@
+NODE_ENV=
+PORT=
+MONGO_URI=
+JWT_SECRET=

package.json

@@ -2,6 +2,7 @@
   "name": "order-service",
   "version": "1.0.0",
   "description": "",
+  "type": "module",
   "main": "index.js",
   "scripts": {
     "test": "echo \"Error: no test specified\" && exit 1"

src/config/env.ts

@@ -0,0 +1,25 @@
+import dotenv from "dotenv";
+
+dotenv.config();
+
+function requireEnv(key: string): string {
+  const value = process.env[key];
+
+  if (!value) {
+    throw new Error(`Environment variable ${key} is required but not set.`);
+  }
+
+  return value;
+}
+
+export const env = {
+  nodeEnv: process.env.NODE_ENV ?? "development",
+  port: Number(process.env.PORT) ?? 3000,
+
+  mongoUri: requireEnv("MONGO_URI"),
+
+  jwtSecret: {
+    secret: requireEnv("JWT_SECRET"),
+    expiresIn: process.env.JWT_EXPIRES_IN ?? "1h",
+  },
+};