fix: critical timestamp parsing and audit fixes

- Fix timestamp conversion: String(epochMs) → new Date(epochMs).toISOString()
  in use-messages-sync, use-comments-sync, use-documents, use-inbox.
  Without this, date comparisons (isEdited, cutoff filters) would fail.
- Fix updated_at: undefined → null in use-inbox to match InboxItem type
- Fix ZeroProvider: skip Zero connection for unauthenticated users
- Clean 30+ stale "Electric SQL" comments in backend Python code

surfsense_backend/app/tasks/celery_tasks/document_tasks.py

@@ -887,7 +887,7 @@ async def _process_file_with_document(
         )
 
         try:
-            # Set status to PROCESSING (shows spinner in UI via ElectricSQL)
+            # Set status to PROCESSING (shows spinner in UI via Zero)
             document.status = DocumentStatus.processing()
             await session.commit()
             logger.info(
@@ -951,7 +951,7 @@ async def _process_file_with_document(
             ):
                 page_limit_error = e.__cause__
 
-            # Mark document as failed (shows error in UI via ElectricSQL)
+            # Mark document as failed (shows error in UI via Zero)
             error_message = str(e)[:500]
             document.status = DocumentStatus.failed(error_message)
             document.updated_at = get_current_timestamp()