Convert GPX/FIT to GeoJSON at upload time, not first view

GpxStore#save now calls geojson() itself right after writing the
file, so the (potentially tens-of-seconds, for a large POI export)
conversion cost is paid during the upload — covered by the frontend's
existing "Uploading…" status — instead of on whichever checkbox
toggle happens to be first, which otherwise reads as a silent hang
indistinguishable from broken. An unparseable file still saves fine;
the error just surfaces on first view same as before.

test/test_gpx_store.rb

@@ -68,20 +68,26 @@ class GpxStoreTest < Minitest::Test
     assert_empty @store.tree["children"].reject { |c| c["name"] == "archive" }
   end
 
-  def test_geojson_is_cached_alongside_the_source_file
+  def test_save_eagerly_caches_the_geojson_conversion
     path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
     cache_path = @store.resolve_file(path) + ".geojson"
-    refute File.file?(cache_path)
-
-    first = @store.geojson(path)
     assert File.file?(cache_path)
 
-    # Corrupt the cache directly to prove the second call reused it instead
-    # of re-parsing the (still-valid) source file.
+    first = @store.geojson(path)
+
+    # Corrupt the cache directly to prove the call above reused it instead of
+    # re-parsing the (still-valid) source file.
     File.write(cache_path, first.sub("waypoint", "cached-waypoint"))
     assert_includes @store.geojson(path), "cached-waypoint"
   end
 
+  def test_save_of_an_unparseable_file_still_succeeds
+    path = @store.save(folder: "", filename: "bad.gpx", io: StringIO.new("not gpx at all"))
+
+    refute File.file?(@store.resolve_file(path) + ".geojson")
+    assert_raises(Maptool::Gpx::ParseError) { @store.geojson(path) }
+  end
+
   def test_geojson_cache_is_invalidated_when_the_file_is_resaved
     path = @store.save(folder: "", filename: "ride.gpx", io: StringIO.new(sample_gpx))
     @store.geojson(path)