Merge origin/main (PR #24 Citation Checker) into ws7-company-search

Conflict resolutions (keep-both throughout):
- AppSidebar.tsx: adopted #24's renderNavItem pattern; the Research group
  now lists both Company Search and Citation Checker (collapsible variant
  from this branch dropped in favour of the merged static group label).
- backend/src/index.ts: both limiters (researchLimiter, citationsLimiter)
  and both mounts (/companies, /citations) kept.
- mikeApi.ts: both the Companies House section and checkCitations kept.
- backend/.env.example: both rate-limit var blocks kept.
- CLAUDE.md: both module-map route entries merged into one row;
  RATE_LIMIT_* count now 16 covering research + citations vars.
- docs/BUILD_LOG.md: both entries kept, this branch's entry first;
  verification updated to the post-merge numbers (vitest 133/133).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>