Cache parsed GPX/FIT GeoJSON to fix large POI-export loads

A several-thousand-waypoint park4night area export re-parses the
whole file with REXML on every toggle — ~2.4s for a 5k-waypoint/3.6MB
sample, so tens of seconds for a 50k-waypoint one, likely the actual
cause behind a "Could not load" report on a 41.5MB export. MapLibre
already clusters the waypoint layer client-side, so the bottleneck was
never rendering — cache the GeoJSON conversion alongside the source
file (invalidated by mtime) instead of a bigger bbox/tiling rework.

app.rb

@@ -193,18 +193,17 @@ module Maptool
 
     get "/api/gpx/file/*" do
       begin
-        path = GPX_STORE.resolve_file(params[:splat].first)
+        body = GPX_STORE.geojson(params[:splat].first)
       rescue GpxStore::InvalidPath
         halt 400
-      end
-      halt 404 unless File.file?(path)
-
-      begin
-        geojson = path.downcase.end_with?(".fit") ? Fit.to_geojson(path) : Gpx.to_geojson(path)
-        json geojson
+      rescue Errno::ENOENT
+        halt 404
       rescue Gpx::ParseError, Fit::ParseError
         halt 422, json(error: "could not parse file")
       end
+
+      content_type :json
+      body
     end
 
     post "/api/gpx/folder" do