Add cloud credential usage inventory for instance references

### Problem\n\nCloud credentials are managed independently from instances. Operators currently cannot see which instances reference a credential, identify unused credentials, detect vendor mismatches, or locate instances that refer to a deleted credential without manually correlating paginated settings data.\n\n### Proposed solution\n\nAdd an on-demand, read-only usage inventory to the cloud credential settings tab. It should:\n\n- load all credential metadata and current instances;\n- classify credentials as used, unused, or vendor-mismatched;\n- list orphan instance references;\n- support vendor/status/text filtering and CSV export;\n- never read or export AccessKey or SecretKey values.\n\n### Scope\n\nThis is an operator visibility feature. It does not change credential CRUD behavior, instance bindings, or introduce enforcement policies.

Start in the cloud credential settings tab and trace how credential metadata and current instances are loaded. The work is done when the read-only inventory classifies used, unused, mismatched, and orphan references, supports vendor/status/text filtering and CSV export, and never exposes AccessKey or SecretKey values.

Written by the indexing model from the issue text.