Use reference URLs for resource visuals

[ewlarson]

Summary

• Use dct_references_s http://schema.org/thumbnailUrl / https://schema.org/thumbnailUrl as a fallback thumbnail source only when no distribution-derived thumbnail source is present.
• Use structured distribution hasMap values first, then legacy dct_references_s http://schema.org/hasMap / https://schema.org/hasMap, before falling through to generated static-map or placeholder/default behavior.
• Carry distribution context into resource static-map presentation and compatibility routes so structured rows remain authoritative.
• Add regression coverage for distribution-first, legacy-reference-second visual source selection.

Validation

• BTAA_SKIP_TEST_DB=1 python -m pytest backend/tests/services/test_static_map_service.py::test_external_static_map_url_reads_schema_has_map_from_dct_references_string backend/tests/services/test_static_map_service.py::test_external_static_map_url_accepts_https_schema_has_map_dict_value backend/tests/services/test_static_map_service.py::test_external_static_map_url_prefers_distribution_has_map_over_dct_references backend/tests/services/test_static_map_service.py::test_external_static_map_url_ignores_non_http_has_map_values backend/tests/services/test_image_service.py::TestImageServiceThumbnailSourceURL::test_get_thumbnail_source_url_prefers_distribution_thumbnail_over_dct_references backend/tests/services/test_image_service.py::TestImageServiceThumbnailURL::test_get_thumbnail_source_url_checks_dct_references_when_distributions_exist backend/tests/api/v1/test_static_map_endpoints.py::TestResourceStaticMapEndpoint::test_resource_static_map_uses_schema_has_map_before_generated_alias backend/tests/api/v1/test_static_map_endpoints.py::TestResourceStaticMapEndpoint::test_resource_static_map_latest_alias_redirects_when_no_schema_has_map backend/tests/api/v1/test_static_map_endpoints.py::TestResourceStaticMapEndpoint::test_resource_static_map_no_cache_uses_schema_has_map backend/tests/api/v1/test_resource_presenter.py::test_resource_presenter_static_map_uses_schema_has_map_without_geometry backend/tests/api/v1/test_resource_presenter.py::test_resource_presenter_static_map_prefers_distribution_has_map_over_legacy_reference backend/tests/api/v1/test_utils.py::TestHotVisualAssetUrls::test_reference_static_map_url_uses_schema_has_map_without_geometry backend/tests/api/v1/test_utils.py::TestHotVisualAssetUrls::test_reference_static_map_url_prefers_distribution_has_map_over_legacy_reference
• python -m ruff check backend/app/services/static_map_service.py backend/app/services/image_service.py backend/app/api/v1/endpoint_modules/resources/static_map.py backend/app/api/v1/utils.py backend/app/api/v1/presenters/resource.py backend/tests/services/test_static_map_service.py backend/tests/services/test_image_service.py backend/tests/api/v1/test_static_map_endpoints.py backend/tests/api/v1/test_resource_presenter.py backend/tests/api/v1/test_utils.py
• python -m py_compile backend/app/services/static_map_service.py backend/app/services/image_service.py backend/app/api/v1/endpoint_modules/resources/static_map.py backend/app/api/v1/utils.py backend/app/api/v1/presenters/resource.py
• git diff --check -- backend/app/services/static_map_service.py backend/app/services/image_service.py backend/app/api/v1/endpoint_modules/resources/static_map.py backend/app/api/v1/utils.py backend/app/api/v1/presenters/resource.py backend/tests/services/test_static_map_service.py backend/tests/services/test_image_service.py backend/tests/api/v1/test_static_map_endpoints.py backend/tests/api/v1/test_resource_presenter.py backend/tests/api/v1/test_utils.py

Note: the broader focused suite reached 164 passed and then failed 5 existing image-service tests because sandboxed Redis access to localhost:6380 was denied with Operation not permitted; the new static-map endpoint/presenter/util tests passed in that run.