♻️(backend) stylistic and consistency changes

Refactored converter services based on PR #1609 review comments:
- Renamed parameter to `data` across all convert methods for consistency
- Replaced recursive call with explicit sequential calls for readability
- Hardcoded CONVERSION_API_SECURE=True in Production class for security
- Removed unused YdocConverter import from viewsets.py
- Updated tests to match new error message wording

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

src/backend/core/tests/test_services_converter_services.py

@@ -22,9 +22,9 @@ def test_auth_header(settings):
 
 
 def test_convert_empty_text():
-    """Should raise ValidationError when text is empty."""
+    """Should raise ValidationError when data is empty."""
     converter = YdocConverter()
-    with pytest.raises(ValidationError, match="Input text cannot be empty"):
+    with pytest.raises(ValidationError, match="Input data cannot be empty"):
         converter.convert("")
 
 
@@ -143,5 +143,5 @@ def test_convert_none_input():
     """Should raise ValidationError when input is None."""
     converter = YdocConverter()
 
-    with pytest.raises(ValidationError, match="Input text cannot be empty"):
+    with pytest.raises(ValidationError, match="Input data cannot be empty"):
         converter.convert(None)