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_web/hooks/use-messages-sync.ts

@@ -30,7 +30,7 @@ export function useMessagesSync(
 			role: msg.role,
 			content: msg.content,
 			author_id: msg.authorId ?? null,
-			created_at: String(msg.createdAt),
+			created_at: new Date(msg.createdAt).toISOString(),
 		}));
 
 		onMessagesUpdateRef.current(mapped);