feat: Add authentication configuration with JWT secret and reorganize environment variables with categorized sections

- Add AuthConfig struct with JWTSecret field to config package
- Add Auth field to Config struct with JSON tag for authentication settings
- Add JWT_SECRET to Auth config with default value "change-me-in-production"
- Replace MONGODB_URI with DATABASE_URL using PostgreSQL connection string in .env.example
- Add categorized sections in .env.example: Database/Server/Authentication/

apps/api-server/.env.example

@@ -1,6 +1,15 @@
-MONGODB_URI=mongodb://localhost:27017/free5gc
+# Database Configuration
+DATABASE_URL=postgres://telecom:password@localhost:5432/telecom_platform?sslmode=disable
 REDIS_URI=redis://localhost:6379
+
+# Server Configuration
 API_PORT=8000
 GIN_MODE=debug
-JWT_SECRET=change-me-in-production
 LOG_LEVEL=info
+
+# Authentication
+JWT_SECRET=change-me-in-production
+
+# External Services
+PROMETHEUS_URL=http://localhost:9090
+KUBERNETES_NAMESPACE=telecom-platform