New Webhook author headers

We now include two optional headers on webhook deliveries to identify the actor that triggered the event:

• x-factorial-author-id
• x-factorial-author-type

The values are resolved from the triggering access when available.

• If the action is company-level, the headers reflect the company as the author.
• If the action is done by an employee, the header reflect the employee as the author.

Besides, we are adding visibility checks: If the subscriber is not allowed to see the author (for example, the author is an employee outside their visibility), the headers are omitted.

This change does not alter webhook payloads or subscription types. It only adds metadata headers when safe to do so.