Notify only internally about the audit result for costs audit

**Is your feature request related to a problem? Please describe.**
Notify only internally about the audit result for costs audit.

**Describe the solution you'd like**
Add a post-processor notifying only internally about the audit result.

**Describe alternatives you've considered**
Get costs via the API

**Additional context**
Costs audit is a global audit, scheduled weekly, by default disabled in the global configuration.
It is only enabled on dev, for ahrefs.com site.
https://github.com/adobe/spacecat-audit-worker/pull/365

Start with the costs audit's global configuration and linked pull request #365 to understand the existing audit flow and any work already underway. Done means the weekly costs-audit result is handled by an internal-only post-processor while the audit remains disabled by default in global configuration.

Written by the indexing model from the issue text.