Introduce study region for exam-oriented heritage search

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Description

The current region field stores UNESCO raw region codes such as AFR, ARB, APA, EUR, and LAC.

These values do not match the mental model of users preparing for the World Heritage Exam Grade 2. For example, a heritage site in the United States may still appear under EUR, which is not intuitive from a study or search UX perspective.

To improve the search experience, we need to introduce an application-specific study_region classification while keeping UNESCO raw region data unchanged.

This parent issue tracks the backend work required to support study_region as a separate classification for exam-oriented filtering and learning UX.

Scope

• Add a study_region column
• Implement a resolver from country name to study_region
• Populate study_region during import
• Backfill existing records safely

Out of scope

• Full database redesign
• Replacing UNESCO raw region data
• Frontend filter migration in this issue