POIs endpoint returns empty features

The /pois endpoint returns HTTP 200 with an empty features array for both bbox and Point+buffer queries in dense city centers (e.g., Heidelberg, Berlin, Paris) and includes an invalid bbox: [NaN, NaN, NaN, NaN] in the response body. Reproducible via API console and client. Example request:

`{"request":"pois","geometry":{"geojson":{"type":"Point","coordinates":[8.6821,49.4106]},"buffer":1500},"limit":50}`

Example response (truncated):

`{"type":"FeatureCollection","bbox":[NaN,NaN,NaN,NaN],"features":[],"information":{...}}`

Looks like a backend regression in POI search and/or bbox generation.

Start by reproducing the Point+buffer and bbox requests against the /pois endpoint using the API console or client. Trace the POI search and bbox generation paths, then verify that dense-city queries return features and that the response bbox contains valid numbers instead of NaN values.

Written by the indexing model from the issue text.