Multi-account support in Self-hosted

[sbaum1994]

Description

Add multiple-account support for self-managed NVCF so operators can separate tenants, teams, or environments beyond nvcf-default.

Definition of Done

• More than one account can be created in a self-managed deployment.
• API keys and scopes are account-specific.
• Cluster registration and function create, deploy, invoke, and list flows work for non-default accounts.
• CLI account selection/configuration and account lifecycle docs exist.
• Tests cover at least two accounts in the same control plane.