Add rake overpass:pois to fetch camper POIs from Overpass

lib/maptool/overpass_pois.rb: queries Overpass for drinking water,
dump stations, LPG refill, toilets, laundromats, supermarkets/
convenience stores, and springs within a bbox, writes GeoJSON. A
dev-time tool (like the offline.pmtiles/terrain.pmtiles generation
steps already documented) — not called by the running app, drop the
output into data/gpx/ or upload it through the Tracks panel like any
other .geojson layer.

Verified against the real Overpass API (88 POIs for central
Bratislava) in addition to the new unit tests (pure to_geojson/
build_query logic, no network in the suite itself). Full suite: 103
runs, 0 failures.

docs/development.adoc

@@ -59,6 +59,19 @@ Terrarium-encoded PNG tiles in an `.mbtiles`), then
 https://github.com/protomaps/go-pmtiles[go-pmtiles], same as
 `data/offline.pmtiles`.
 
+== Camper-relevant POIs (Overpass)
+
+`rake overpass:pois[south,west,north,east,out.geojson]` queries
+https://overpass-api.de[Overpass] for drinking water, dump stations,
+LPG refill, public toilets, laundromats, supermarkets/convenience
+stores, and springs within a bbox, and writes the result as a
+`.geojson` file (`lib/maptool/overpass_pois.rb`). A dev-time tool, not
+something the running app calls itself — drop the output into
+`data/gpx/` (or upload it through the Tracks panel) like any other
+`.geojson` layer once it's generated. Nodes only (nothing that's
+mapped as a way/relation, e.g. a supermarket traced as a building
+outline rather than a single point).
+
 == GNSS position (gpsd)
 
 The "GNSS" control needs https://gpsd.io[gpsd] running and pointed at