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-comments-sync.ts

@@ -203,8 +203,8 @@ export function useCommentsSync(threadId: number | null) {
 			parent_id: c.parentId ?? null,
 			author_id: c.authorId ?? null,
 			content: c.content,
-			created_at: String(c.createdAt),
-			updated_at: String(c.updatedAt),
+			created_at: new Date(c.createdAt).toISOString(),
+			updated_at: new Date(c.updatedAt).toISOString(),
 		}));
 
 		updateReactQueryCache(rows);