Improve Onboard API with Optional OpenAI Key and Transactional Consistency

[avirajsingh7]

The current Onboard API has two main problems:

• openai_api_key is not properly supported as an optional parameter, even though Kaapi depends on it when provided.
• The onboarding process may create immature/partial entries (organization/project) if validation fails later (e.g., password too short). This happens because each CRUD operation commits independently.