fix: add original_filename field to DocumentMetadata

Add optional original_filename field to DocumentMetadata dataclass
to properly store the original filename when files are converted
(e.g., Office → PDF). This ensures the field is included in to_dict()
output for JSON serialization.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/models/unified_document.py

@@ -406,6 +406,7 @@ class DocumentMetadata:
     creator: Optional[str] = None
     creation_date: Optional[datetime] = None
     modification_date: Optional[datetime] = None
+    original_filename: Optional[str] = None  # Original filename before conversion (e.g., Office → PDF)
 
     def to_dict(self) -> Dict[str, Any]:
         """Convert to dictionary for JSON serialization"""
@@ -421,7 +422,7 @@ class DocumentMetadata:
         # Add optional fields if present
         optional_fields = [
             "language", "title", "author", "subject",
-            "keywords", "producer", "creator"
+            "keywords", "producer", "creator", "original_filename"
         ]
         for field in optional_fields:
             value = getattr(self, field)