This fixes a critical data loss issue where users editing the same memo
on multiple devices would overwrite each other's changes due to aggressive
browser caching, particularly in Chromium-based browsers and PWAs.

Changes:
- Backend: Add Cache-Control headers to all API responses to prevent
  browser HTTP caching
- Frontend: Force fresh fetch from server when opening memo editor by
  invalidating React Query cache
- Frontend: Reduce memo query staleTime from 60s to 10s for better
  collaborative editing support

Fixes #5470