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
+}

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

@@ -0,0 +1,7 @@
+namespace DigitalData.EmailProfiler.Domain.Enums;
+
+public enum AuthenticationType
+{
+    UsernamePassword = 1,
+    OAuth2 = 2
+}

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

@@ -0,0 +1,11 @@
+namespace DigitalData.EmailProfiler.Domain.Enums;
+
+public enum EmailStatus
+{
+    Pending = 1,
+    Processing = 2,
+    Processed = 3,
+    Failed = 4,
+    PartiallyProcessed = 5,
+    Rejected = 6
+}

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

@@ -0,0 +1,16 @@
+namespace DigitalData.EmailProfiler.Domain.Enums;
+
+public enum ErrorCode
+{
+    None = 0,
+    NoAttachments = 10001,
+    SenderValidationFailed = 10002,
+    EmbeddedFileAttachmentCorrupt = 10003,
+    NormalFileAttachmentCorrupt = 10004,
+    PdfStructureInvalid = 10005,
+    ImapConnectionFailed = 10006,
+    WindreamImportFailed = 10007,
+    DiskSpaceInsufficient = 10008,
+    DuplicateMessageId = 10009,
+    AttachmentExtractionFailed = 10010
+}

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

@@ -0,0 +1,8 @@
+namespace DigitalData.EmailProfiler.Domain.Enums;
+
+public enum ProcessType
+{
+    ProcessManager = 1,      // Easy Approval workflow
+    AttachmentSniffer = 2,   // General attachment extraction
+    ZugFeRDParser = 3        // Electronic invoice processing
+}