Add verification_level enum column to ai_catalog_items table

Description:

Introduce a new verification_level column to the ai_catalog_items table to align with the structure of the catalog_resources table.

Details:

• Add a new verification_level enum column to ai_catalog_items.
• Set default value to 0 (unverified).
• Ensure the column is indexed to support efficient lookups and filtering.
• Add a scope to the AiCatalogItem model to filter records by verification_level.

This change is foundational for enabling verification-level tracking in AI Catalog Items, similar to CI Catalog components.