fix: improve dev backend startup reliability

[UttamPachauri]

Summary

This PR improves the reliability of the local development startup script by adding validation, configurable startup options, and better process management.

Changes Made

• Added configurable ports using MAIN_API_PORT and LEVELS_API_PORT environment variables.
• Added support for --main-only and --levels-only startup flags.
• Added validation to ensure the levels backend contains a valid package.json before startup.
• Refactored service startup into a reusable spawnService() helper.
• Improved startup logging for better developer experience.
• Added graceful shutdown protection to prevent duplicate termination.
• Improved error reporting when development services fail to start.

Benefits

• Makes the development workflow more reliable.
• Prevents invalid startup configurations.
• Improves debugging with clearer startup and error messages.
• Allows developers to start only the required backend service.
• Reduces duplicate startup and shutdown logic.

Testing

• Verified default startup flow.
• Tested custom port configuration using environment variables.
• Tested --main-only and --levels-only flags.
• Verified validation when the levels backend configuration is invalid.
• Confirmed existing startup behavior remains unchanged.