public class OutboxEvent
{
    public Guid Id { get; set; }
    public string Topic { get; set; } = null!;
    public string Payload { get; set; } = null!;  // JSON string
    public string? PartitionKey { get; set; }   // encounter_id for all clinical events
    public DateTimeOffset CreatedAt { get; set; }
    public DateTimeOffset? ProcessedAt { get; set; }
}