Bug: Board candidates missing-resource path returns soft-404 (HTTP 200)

Issue Description
The board candidates page renders a "Board not found" UI with 404 messaging, but the route still returns HTTP 200. This creates a soft-404 behavior.

Observed in:
- `frontend/src/app/board/[year]/candidates/page.tsx`

Current behavior:
- Missing board data returns ``
- HTTP response remains 200 instead of 404.

This impacts SEO/indexing and makes monitoring/caching semantics incorrect for genuinely missing resources.

Proposed Fix
For true missing-resource cases, return a real Next.js 404 response by using `notFound()` instead of only rendering 404-like UI.

Are you going to work on implementing this?

- [x] Yes
- [ ] No