feat(domain): add domain enumerations

- ErrorCode: All error codes from legacy system (10001-10010)
- ProcessType: Email process types (ProcessManager, AttachmentSniffer, ZugFeRDParser)
- AuthenticationType: Authentication methods (UsernamePassword, OAuth2)
- EmailStatus: Email processing status tracking
- AttachmentStatus: Attachment validation status

These enums maintain compatibility with the legacy VB.NET system.

src/DigitalData.EmailProfiler.Domain/Enums/AttachmentStatus.cs

@@ -0,0 +1,9 @@
+namespace DigitalData.EmailProfiler.Domain.Enums;
+
+public enum AttachmentStatus
+{
+    Pending = 1,
+    Valid = 2,
+    Corrupt = 3,
+    Skipped = 4
+}