Add import/export support for AI settings and provider configuration

[jeffpaul]

Provide a way to import and export AI settings and provider configurations to support portability across environments. This is especially useful for agencies, hosts, and multi-site workflows.

Security and credential handling must be carefully considered.

Acceptance criteria:

• Settings can be exported in a portable format
• Settings can be imported safely and predictably
• Sensitive credentials are handled securely
• Feature is clearly documented
• Consider support for env variables for settings/provider configs