Refactor code structure for improved readability and maintainability

.env.example

@@ -0,0 +1,15 @@
+# PostgreSQL + MongoDB
+DATABASE_URL=postgresql://reader:reader@localhost:5432/reader
+MONGODB_URI=mongodb://localhost:27017/reader
+
+# Auth / OAuth
+NEXTAUTH_SECRET=replace-with-strong-secret
+MOBILE_JWT_SECRET=replace-with-strong-secret
+# Comma-separated allowed Google OAuth client IDs (web + android if needed)
+GOOGLE_CLIENT_ID=web-client-id.apps.googleusercontent.com,android-client-id.apps.googleusercontent.com
+
+# CORS (comma-separated), * for all in local dev
+CORS_ORIGINS=http://localhost:3000,http://127.0.0.1:3000
+
+# Environment label
+APP_ENV=development