fix(python): remove datetime.UTC usage for Python 3.10 compatibility

Replace datetime.datetime.now(datetime.UTC) with datetime.datetime.now()
to fix compatibility issues with Python 3.10 and earlier versions.

datetime.UTC is only available in Python 3.11+, causing 500 errors
in production environment.

Files fixed:
- app/module/dataset/service/pdf_extract.py
- app/module/generation/service/export_service.py

runtime/datamate-python/app/module/dataset/service/pdf_extract.py

@@ -278,7 +278,7 @@ class PdfTextExtractService:
             file_type=derived_file_type,
             file_size=file_size,
             dataset_filemetadata=metadata,
-            last_access_time=datetime.datetime.now(datetime.UTC),
+            last_access_time=datetime.datetime.now(),
         )
         self.db.add(record)
         dataset.file_count = (dataset.file_count or 0) + 1

runtime/datamate-python/app/module/generation/service/export_service.py

@@ -87,7 +87,7 @@ class SynthesisDatasetExporter:
                 file_path=file_path,
                 file_type="jsonl",
                 file_size=file_size,
-                last_access_time=datetime.datetime.now(datetime.UTC),
+                last_access_time=datetime.datetime.now(),
             )
             self._db.add(df)
             created_files.append(df)