Ash preparation that filters event logs by the actor's site_id.

EventLog stores site_id inside JSONB metadata/data fields rather than as a top-level column, so standard Ash multitenancy doesn't apply. This preparation reads the actor's site_id and applies a fragment filter.

• Master users (site_id: nil) see all events — no filter applied.
• Tenant users see only events matching their site_id in metadata or data, AND only events created by system (no actor) or by tenant users (not master users).
• Anything else, a missing actor included, sees no event.