feature: Outbox Relay and Kafka Pipeline

VigilCareClinicalAPI/Domains/Entities/OutboxEvent.cs

@@ -3,6 +3,7 @@ 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; }
 }