Add Allmaps map overlay search facet

[ewlarson]

Summary

Adds the missing Map Overlay search facet so users can filter search results down to Allmaps-annotated maps from the main results page.

What changed

• Indexes b1g_georeferenced_allmaps_b from resource_allmaps.annotated for Elasticsearch documents.
• Adds the field to mappings, default search aggregations, facet endpoint validation, and legacy alias handling via map_overlay_agg.
• Adds the frontend Map Overlay facet label/order and boolean value handling for true / false buckets.
• Adds focused backend and frontend coverage for the new facet path.

Notes

Existing deployments will need a reindex before the facet counts appear, since the field is populated at index time.

Refs #72.

Validation

• /Users/ewlarson/.pyenv/versions/3.13.1/bin/python -m pytest backend/tests/elasticsearch/test_index.py backend/tests/elasticsearch/test_facet_functions.py backend/tests/elasticsearch/test_mappings.py backend/tests/services/test_search_service.py backend/tests/api/v1/test_strong_params.py backend/tests/api/v1/test_facet_endpoint.py
• npm test -- --run src/__tests__/utils/facetLabels.test.ts src/__tests__/utils/facetDisplay.test.ts src/__tests__/utils/searchParams.test.ts src/__tests__/components/FacetList.test.tsx
• ./node_modules/.bin/eslint src/constants/facets.ts src/constants/fieldLabels.ts src/services/api.ts src/utils/facetDisplay.ts src/utils/facetLabels.ts src/utils/searchParams.ts src/__tests__/utils/facetLabels.test.ts src/__tests__/utils/facetDisplay.test.ts src/__tests__/utils/searchParams.test.ts src/__tests__/components/FacetList.test.tsx
• /Users/ewlarson/.pyenv/versions/3.13.1/bin/python -m ruff check backend/app/api/v1/strong_params.py backend/app/elasticsearch/index.py backend/app/elasticsearch/mappings.py backend/app/elasticsearch/search.py backend/app/services/search_service.py backend/scripts/reindex_atomic.py backend/tests/api/v1/test_facet_endpoint.py backend/tests/api/v1/test_strong_params.py backend/tests/elasticsearch/test_facet_functions.py backend/tests/elasticsearch/test_mappings.py backend/tests/elasticsearch/test_index.py backend/tests/services/test_search_service.py
• npm run build