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.

test/test_overpass_pois.rb

@@ -0,0 +1,68 @@
+require_relative "test_helper"
+require "maptool/overpass_pois"
+
+# No real network calls — to_geojson is pure (canned Overpass response ->
+# GeoJSON), and that's the part worth protecting from regressions. fetch()
+# itself is a thin Net::HTTP.post_form + JSON.parse wrapper, same as
+# LiveTrackClient's own fetch_html, not independently tested for the same
+# reason that one isn't.
+class OverpassPoisTest < Minitest::Test
+  def test_to_geojson_converts_matching_node_elements
+    response = {
+      "elements" => [
+        { "type" => "node", "lat" => 48.1, "lon" => 17.2, "tags" => { "amenity" => "drinking_water" } },
+        { "type" => "node", "lat" => 48.3, "lon" => 17.4, "tags" => { "amenity" => "toilets", "name" => "Rest area" } },
+      ],
+    }
+
+    geojson = Maptool::OverpassPois.to_geojson(response)
+
+    assert_equal "FeatureCollection", geojson["type"]
+    assert_equal 2, geojson["features"].size
+    water = geojson["features"].find { |f| f["properties"]["category"] == "Drinking water" }
+    assert_equal [17.2, 48.1], water["geometry"]["coordinates"]
+    assert_equal "Drinking water", water["properties"]["name"], "falls back to the category when the node has no name tag"
+  end
+
+  def test_to_geojson_uses_the_node_name_when_present
+    response = { "elements" => [{ "type" => "node", "lat" => 48.3, "lon" => 17.4, "tags" => { "amenity" => "toilets", "name" => "Rest area" } }] }
+
+    feature = Maptool::OverpassPois.to_geojson(response)["features"].first
+
+    assert_equal "Rest area", feature["properties"]["name"]
+  end
+
+  def test_to_geojson_skips_ways_and_relations
+    response = { "elements" => [{ "type" => "way", "tags" => { "amenity" => "drinking_water" } }] }
+
+    assert_empty Maptool::OverpassPois.to_geojson(response)["features"]
+  end
+
+  def test_to_geojson_skips_nodes_matching_no_known_category
+    response = { "elements" => [{ "type" => "node", "lat" => 48.1, "lon" => 17.2, "tags" => { "amenity" => "restaurant" } }] }
+
+    assert_empty Maptool::OverpassPois.to_geojson(response)["features"]
+  end
+
+  def test_lpg_matches_either_fuel_stations_with_the_lpg_tag_or_gas_bottle_shops
+    response = {
+      "elements" => [
+        { "type" => "node", "lat" => 1, "lon" => 1, "tags" => { "amenity" => "fuel", "fuel:lpg" => "yes" } },
+        { "type" => "node", "lat" => 2, "lon" => 2, "tags" => { "shop" => "gas" } },
+        { "type" => "node", "lat" => 3, "lon" => 3, "tags" => { "amenity" => "fuel" } }, # no lpg tag - not a match
+      ],
+    }
+
+    categories = Maptool::OverpassPois.to_geojson(response)["features"].map { |f| f["properties"]["category"] }
+
+    assert_equal ["LPG refill", "LPG refill"], categories
+  end
+
+  def test_build_query_includes_the_bbox_and_every_category_filter
+    query = Maptool::OverpassPois.build_query([48.0, 17.0, 49.0, 18.0])
+
+    assert_includes query, "48.0,17.0,49.0,18.0"
+    assert_includes query, %(["amenity"="drinking_water"])
+    assert_includes query, %(["natural"="spring"])
+  end
+end