Complete Phase 2: Domain Layer implementation

Updated ROADMAP.md to mark Phase 2 as completed and added detailed descriptions of completed tasks. Introduced three new value objects (`Base64String`, `TenantId`, and `PdfMetadata`) in the `DocumentOperator.Domain.Models.ValueObjects` namespace. These classes ensure type safety, immutability, and encapsulated validation.

- `Base64String`: Handles Base64 string creation, validation, and conversion.
- `TenantId`: Represents a tenant identifier with validation and normalization.
- `PdfMetadata`: Represents PDF metadata with computed properties.

Updated `DocumentOperator.Domain.csproj` to reflect the addition of these value objects. The project is now ready to begin Phase 3 (Infrastructure Layer).

DocumentOperator.Domain/Models/ValueObjects/Base64String.cs

@@ -0,0 +1,59 @@
+namespace DocumentOperator.Domain.Models.ValueObjects;
+
+public sealed class Base64String
+{
+    public string Value { get; }
+
+    private Base64String(string value)
+    {
+        Value = value;
+    }
+
+    public static Base64String Create(string value)
+    {
+        if (string.IsNullOrWhiteSpace(value))
+            throw new Common.Exceptions.DomainValidationException("Base64 string cannot be empty.");
+
+        // Validierung: Ist es gültiges Base64?
+        try
+        {
+            Convert.FromBase64String(value);
+        }
+        catch (FormatException)
+        {
+            throw new Common.Exceptions.DomainValidationException("Invalid Base64 format.");
+        }
+
+        return new Base64String(value);
+    }
+
+    public static Base64String FromByteArray(byte[] bytes)
+    {
+        if (bytes == null || bytes.Length == 0)
+            throw new Common.Exceptions.DomainValidationException("Byte array cannot be null or empty.");
+
+        var base64 = Convert.ToBase64String(bytes);
+        return new Base64String(base64);
+    }
+
+    public byte[] ToByteArray()
+    {
+        return Convert.FromBase64String(Value);
+    }
+
+    public override string ToString() => Value;
+
+    // Equality (wichtig für Value Objects!)
+    public override bool Equals(object? obj)
+    {
+        if (obj is not Base64String other)
+            return false;
+
+        return Value == other.Value;
+    }
+
+    public override int GetHashCode()
+    {
+        return Value.GetHashCode();
+    }
+}