feat(S2-b): connect durable MQTT sync to capture workflow

FieldLogger/Models/LoggedPoint.cs

@@ -69,7 +69,13 @@ public sealed class LoggedPoint
     public string GpsRawSentence { get; set; } = "";
     public string GpsSerialNumber { get; set; } = "";
 
-    /// <summary>Reserved for MQTT sync.</summary>
+    /// <summary>Stable UUIDv7 used for MQTT retries and cloud idempotency.</summary>
+    [Indexed(Unique = true)]
+    public string? SyncPointId { get; set; }
+
+    /// <summary>Machine-readable terminal/configuration error; null while pending or after success.</summary>
+    public string? SyncError { get; set; }
+
     public bool Synced { get; set; }
 
     [Ignore]