Export audit log as file

### Please agree to the following

- [X] I have searched [existing issues](https://github.com/cryptomator/hub/issues?q=) for duplicates
- [X] I agree to follow this project's [Code of Conduct](https://github.com/cryptomator/hub/blob/develop/.github/CODE_OF_CONDUCT.md)

### Summary

Export audit log e.g. for automated analysis

### Motivation

As an admin, I currently have to manually check the audit log regularly. I would like to automate this task, e.g. by pre-filtering audit events via a script. Therefore an export in a structured format (e.g. csv) would be helpful.

### Considered Alternatives

_No response_

### Anything else?

_No response_

The issue names no files, tests, or entry points. Start by locating the existing audit-log implementation and its admin access path; clarify the structured format and export interface, then verify that an administrator can retrieve filtered events for automated analysis.

Written by the indexing model from the issue text.