refactor(per): consolidate PER dashboard data endpoints

[arunissun]

Summary

• Consolidate the PER map and dashboard data query/serialization logic.
• Return stable latest-country map results while retaining complete PER process history.
• Add read-only response caching and bounded-query coverage for both endpoints.
• Add focused regression tests for response behavior, ordering, cache use, and realistic related data.

Validation

• Black, isort, and Flake8 passed.
• Django migration check passed with no changes detected.
• Focused PER endpoint tests: 9 passed.
• OpenAPI schema matches the committed artifact.
• Full API suite: 324 passed, 4 skipped.

Scope

No migrations, route changes, frontend changes, or unrelated cleanup.