Add IPdfProcessor interface and update ROADMAP.md

The `IPdfProcessor` interface was added to the `Application/Common/Interfaces/` directory. It includes the `ValidateAsync` method for validating PDFs and extracting metadata, with proper XML documentation and dependency on domain value objects.

Updated `ROADMAP.md` to mark Step 3.1 as completed, detailing the creation of the `IPdfProcessor` interface and its implementation status.

Removed the `<Folder Include="Common\Interfaces\" />` entry from `DocumentOperator.Application.csproj` to reflect the transition from a placeholder folder structure to actual implementation.

DocumentOperator.API/ROADMAP.md

@@ -851,18 +851,21 @@ DocumentOperator.Tests/
 
 ---
 
-#### ?? Step 3.1: IPdfProcessor Interface erstellen - **NEXT**
+#### ? Step 3.1: IPdfProcessor Interface erstellen - **COMPLETED**
 
 **Aufgabe:** Abstraction für PDF-Operationen
 
-**Was du erstellen wirst:**
-- **Wo:** `Application/Common/Interfaces/IPdfProcessor.cs`
+**Was du erstellt hast:**
+- **Wo:** `Application/Common/Interfaces/IPdfProcessor.cs` ?
 - **Inhalt:**
   ```csharp
+  using DocumentOperator.Domain.Models.ValueObjects;
+
+  namespace DocumentOperator.Application.Common.Interfaces;
+
   public interface IPdfProcessor
   {
       Task<PdfMetadata> ValidateAsync(byte[] pdfBytes);
-      // Weitere Methoden später (YAGNI!)
   }
   ```
 
@@ -871,9 +874,15 @@ DocumentOperator.Tests/
 - Infrastructure implementiert
 - TDD: Test ? Interface ? Implementation
 
+**Status:** ? **COMPLETED** (17.01.2025)
+- ? Interface erstellt mit XML Comments
+- ? Using Statement korrekt
+- ? Namespace korrekt
+- ? Build erfolgreich
+
 ---
 
-#### ? Step 3.2: DevExpressPdfProcessor implementieren (mit TDD!)
+#### ?? Step 3.2: DevExpressPdfProcessor implementieren (mit TDD!) - **NEXT**
 
 **Aufgabe:** DevExpress Integration