Refactoring API

[zigzagdev]

Overview

Tracks all API performance and code quality improvements identified through query analysis.
The primary driver is slow page load reported by users — root causes have been pinpointed in query patterns and missing database indexes.

Sub-Issues

🔴 Priority: Critical

• Add index on site_state_parties.world_heritage_site_id
• Remove unnecessary images eager load from list endpoint

🟡 Priority: High

• Add index on world_heritage_sites.study_region
• Add HTTP Cache-Control headers to list/detail responses

🟢 Priority: Medium

• Remove descriptions eager load from list endpoint (pending frontend confirmation)

Acceptance Criteria

• GET /api/world-heritages responds in < 300 ms (p95)
• GET /api/world-heritages/{id} responds in < 200 ms (p95)