Eliminate N+1 queries from filtered GeoJSON responses

[andreagrandi]

Context

Filtered and bounds-based map GeoJSON responses deferred each library description even though serialization reads that field. This caused one additional database query per serialized library and made request cost grow with result size.

What changed

• Include description in the filtered GeoJSON queryset projection.
• Add regression coverage for both search-filtered and bounds-based requests.
• Assert that descriptions remain unchanged and serialization uses a fixed two-query shape.

Notes / constraints

• The GeoJSON response schema and existing filter behavior are unchanged.
• No API documentation update is needed because this is a query-loading fix with no contract change.

Testing

• nox -s tests
• direnv exec . python manage.py check
• git diff --check
• Standalone headed Chromium verification of a city-filtered map request, a bounds-only request, and a marker popup using the returned description; no browser page errors observed.

Closes #178