feat: allow for exporting better audit logs from plik

[iayouaz]

Actual plik logs are not enough for a proper abuse system.
This adds a new logger that can be enabled by specifying a path in AuditLogPath in the config file.
The logger is using a standard timestamp with timezone and milliseconds.
The logs are easier to parse.

Log example:

[2025-07-23T04:23:33.171-04:00] user_id: local:username, user_login: username, user_email: , file_name: a.test, file_md5: f256e2aaafbf7358fdde734c1f263fb2, file_size: 161, mime_type: text/html; charset=utf-8, upload_id: qbhoDOt1RqmtW6jt, upload_login: , upload_ttl: 2592000, upload_upload_token: Yt6JmfsTSyh3ZUrav0zAETcyJFekPCU6, upload_ip_src: 127.0.0.1, method: GET, uri: /file/qbhoDOt1RqmtW6jt/5fLEnTb4qrx3JwlN/a, status_code: 301, ip_src: 127.0.0.1

The logs should not leak any secret like password or private token nor the file content.