refactor: rename upload_to_s3 to process_document

- Rename upload_to_s3.py to process_document.py to better reflect its purpose
- Update all import statements across the codebase to use new module name
- Remove S3-specific code and references
- Keep the core document processing logic intact
- Update docstrings and comments to reflect new functionality

This change is part of removing AWS S3 dependencies and simplifying the
document processing pipeline.

app/utils.py

@@ -1,5 +1,7 @@
 # app/utils.py
 import hashlib
+from app.database import SessionLocal
+from app.models import ProcessingLog
 
 def hash_file(filepath, chunk_size=65536):
     """
@@ -14,3 +16,18 @@ def hash_file(filepath, chunk_size=65536):
                 break
             sha256.update(data)
     return sha256.hexdigest()
+
+def log_task_progress(task_id, step_name, status, message=None, file_id=None):
+    """
+    Logs the progress of a Celery task to the database.
+    """
+    with SessionLocal() as db:
+        log_entry = ProcessingLog(
+            task_id=task_id,
+            step_name=step_name,
+            status=status,
+            message=message,
+            file_id=file_id,
+        )
+        db.add(log_entry)
+        db.commit()