File-Upload: Attachments-Tabelle, Base64-Speicherung, Frontend mit Drag&Drop/Paste/Preview, Error-Handling Fix

Models/MessageRecord.cs

@@ -11,5 +11,17 @@ public class MessageRecord
     public string ToolCallId { get; set; }   // tool_use id
     public bool IsError { get; set; }
     public int Iteration { get; set; }       // welche Loop-Iteration
+    public int AttachmentCount { get; set; } // Anzahl angehängter Dateien
+    public string CreatedAt { get; set; }
+}
+
+public class AttachmentRecord
+{
+    public long Id { get; set; }
+    public long MessageId { get; set; }      // FK auf messages.id
+    public string Type { get; set; }         // image, document
+    public string MediaType { get; set; }    // image/png, application/pdf, etc.
+    public string FileName { get; set; }     // Original-Dateiname (optional)
+    public string Data { get; set; }         // Base64-encoded Inhalt
     public string CreatedAt { get; set; }
 }