Remove ProjectUser model and project-level user management

[avirajsingh7]

The ProjectUser model was part of the previous scope, but we currently don't see a use for it in the current implementation. If we need to implement project-user relationships in the future, it will be from an RBAC perspective with a different design approach.

• The ProjectUser model and related infrastructure is unused and adds unnecessary complexity
• Current project-user management doesn't align with future RBAC plans
• Maintaining unused code increases technical debt and cognitive overhead