feat(MailConfig): Schnittstelle zur Mail-Konfiguration über appsettings erstellt.

EnvelopeGenerator.Application/Configurations/MailConfig.cs

@@ -0,0 +1,7 @@
+namespace EnvelopeGenerator.Application.Configurations
+{
+    public class MailConfig
+    {
+        public required Dictionary<string, string> Placeholders { get; init; } = new();
+    }
+}