Performance: Improve API response speed (slow load — critical)

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Goal

Users reported that the app loads slowly. This is the most critical UX issue to address.
This parent issue tracks all backend performance improvements needed to reduce API response time.

Sub-Issues

• Add database indexes on commonly filtered/sorted columns
• Optimize eager loading in list and detail endpoints (reduce over-fetching)
• Add HTTP Cache-Control headers to list/detail responses
• Trim SELECT columns in list query to minimum required for the list view
• Investigate and fix N+1 query patterns in QueryService

Out of Scope (Frontend repo)

• Image lazy loading / progressive loading
• React bundle size / code splitting
• Client-side caching (SWR / React Query)

Acceptance Criteria

• List endpoint (GET /api/world-heritages) responds in < 300 ms (p95) under typical load
• Detail endpoint (GET /api/world-heritages/{id}) responds in < 200 ms (p95)