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.

TODO.adoc

@@ -12,7 +12,7 @@ independence from connectivity.
 * [x] Shaded relief / DEM layer — hillshade toggle in the sidebar, sourced from public Terrarium tiles online; reads `data/terrain.pmtiles` for offline if one's been generated (see docs/development.adoc), otherwise the toggle is online-only for now
 ** [ ] Terrain slope readout for parking (numeric, not just visual shading)
 * [ ] Contour lines from OSM for hikes and mountain passes
-* [ ] Overpass/OSM scripts to extract POIs (drinking water, dump stations, LPG, public toilets, laundromats, shops, springs)
+* [x] Overpass/OSM scripts to extract POIs (drinking water, dump stations, LPG, public toilets, laundromats, shops, springs) — `rake overpass:pois`, see docs/development.adoc; writes a `.geojson` to drop in `data/gpx/` or upload, not called by the running app itself
 * [ ] Import Park4Night / iOverlander / Freecampsites data
 * [ ] Kiwix/ZIM offline Wikipedia linked to POI GPS coordinates
 * [x] Cache weather forecast when temporarily connected — Open-Meteo 3-day forecast in the Radar flyout, last successful fetch cached in localStorage as a fallback when offline