Audit dashboard lacks an exact operator filter

## Problem

The audit dashboard has an operator column and reports the number of unique operators, but it does not provide an operator filter. To inspect one operator's activity, users must type the operator name into the general search box. That search matches `operator`, `resource_name`, and `detail` together, so a common token can return many unrelated records and cannot be combined cleanly with the existing operation, resource, cluster, date, and result filters.

The filter-options endpoint currently returns operation types, resource types, cluster IDs, and results, but not the distinct operator values already persisted in the audit table.

## Expected behavior

- return the distinct persisted operator values from the audit filter-options API;
- add an operator filter to the audit page alongside the other filters;
- apply the operator filter as an exact match to list, summary, and export queries so all three views stay consistent;
- keep the free-text search unchanged for broad searches;
- include regression coverage for the API, repository predicate, and UI request.

Start with the audit filter-options API, audit repository predicate, audit page, and the list, summary, and export query paths mentioned in the issue. Add the distinct persisted operator values and carry an exact operator filter through all three views while leaving free-text search unchanged. Add regression coverage for the API, repository predicate, and UI request.

Written by the indexing model from the issue text.