feat(pdfa): add FreeTSA timestamping, per-provider folder overrides, individual upload toggles

- Add RFC 3161 timestamping via FreeTSA (PDFA_TIMESTAMP_ENABLED, PDFA_TIMESTAMP_URL)
- Replace PDFA_UPLOAD_TO_PROVIDERS with individual PDFA_UPLOAD_ORIGINAL and PDFA_UPLOAD_PROCESSED
- Add PDFA_UPLOAD_FOLDER setting for per-provider subfolder configuration
- Add GOOGLE_DRIVE_PDFA_FOLDER_ID for Google Drive-specific folder override
- Add folder_override parameter to all 8 folder-using upload tasks
- Add folder_overrides dict parameter to send_to_all_destinations
- Add _compute_pdfa_folder_overrides() and _timestamp_file() helpers
- Expand tests to 26 (timestamping, folder overrides, individual toggles)
- Update docs/ConfigurationGuide.md and .env.demo

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/tasks/upload_to_dropbox.py

@@ -103,7 +103,7 @@ def get_dropbox_client():
 
 
 @celery.task(base=UploadTaskWithRetry, bind=True)
-def upload_to_dropbox(self, file_path: str, file_id: int = None):
+def upload_to_dropbox(self, file_path: str, file_id: int = None, folder_override: str = None):
     """
     Upload a file to Dropbox.
 
@@ -147,7 +147,7 @@ def upload_to_dropbox(self, file_path: str, file_id: int = None):
         dbx = get_dropbox_client()
 
         # Calculate remote path based on local file structure
-        remote_base = settings.dropbox_folder or ""
+        remote_base = folder_override if folder_override is not None else (settings.dropbox_folder or "")
         remote_path = extract_remote_path(file_path, settings.workdir, remote_base)
 
         # Function to check if file exists in Dropbox