Add an import plugin system, with the Overpass POI query as its first plugin

A plugin is a directory under plugins/ with a plugin.json manifest
({"label": "...", "command": "./run.sh"}) plus a script that reads
--south/--west/--north/--east and prints a GeoJSON FeatureCollection
to stdout (lib/maptool/plugins.rb) — that's the entire interface a
plugin needs to know about this app. The sidebar's new import icon
(logged in only, GET /api/plugins + POST /api/plugins/:name/run) lists
whatever's found and runs one for the current map viewport, saving its
output as a new .geojson file the same way an upload would be.

The point: a scraper for a site whose ToS forbids that (Park4Night,
iOverlander, Freecampsites) doesn't need to live in this public repo
to work here — its own directory just goes under plugins/ locally and
is never git add'ed, no special-casing needed anywhere.

plugins/overpass-pois wraps the existing rake overpass:pois query as
the first, in-repo, reference example of the contract.

Also fills in four modules (auth.js, locate.js, measure.js, notes.js)
missing from layout.erb's cache-busting importmap alongside adding
plugins.js's own entry — a pre-existing gap that meant they could keep
serving a stale cached copy across a deploy despite the importmap
existing specifically to prevent that.

test/test_plugins.rb

@@ -0,0 +1,55 @@
+require_relative "test_helper"
+require "maptool/plugins"
+require "tmpdir"
+require "fileutils"
+
+class PluginsTest < Minitest::Test
+  def setup
+    @dir = Dir.mktmpdir("maptool-plugins-test")
+    @plugin_dir = File.join(@dir, "hello")
+    FileUtils.mkdir_p(@plugin_dir)
+    File.write(File.join(@plugin_dir, "plugin.json"), JSON.generate(label: "Hello", command: "./run.sh"))
+    File.write(File.join(@plugin_dir, "run.sh"), "#!/bin/sh\necho '{\"type\":\"FeatureCollection\",\"features\":[]}'\n")
+    File.chmod(0o755, File.join(@plugin_dir, "run.sh"))
+  end
+
+  def teardown
+    FileUtils.remove_entry(@dir)
+  end
+
+  def test_list_finds_plugins_by_their_manifest
+    assert_equal [{ "name" => "hello", "label" => "Hello" }], Maptool::Plugins.list(@dir)
+  end
+
+  def test_list_is_empty_for_a_directory_with_no_plugins
+    assert_empty Maptool::Plugins.list(Dir.mktmpdir)
+  end
+
+  def test_run_returns_the_plugins_geojson_output
+    result = Maptool::Plugins.run(@dir, "hello", [1, 2, 3, 4])
+    assert_equal({ "type" => "FeatureCollection", "features" => [] }, result)
+  end
+
+  def test_run_raises_for_an_unknown_plugin
+    assert_raises(Maptool::Plugins::RunError) { Maptool::Plugins.run(@dir, "nope", [1, 2, 3, 4]) }
+  end
+
+  # A directory-name-shaped ".." would otherwise resolve one level above
+  # dir via File.join — the same class of path-escape GpxStore guards
+  # against for uploaded file paths.
+  def test_run_rejects_a_path_traversal_attempt_as_the_name
+    assert_raises(Maptool::Plugins::RunError) { Maptool::Plugins.run(@dir, "..", [1, 2, 3, 4]) }
+  end
+
+  def test_run_raises_when_the_script_exits_non_zero
+    File.write(File.join(@plugin_dir, "run.sh"), "#!/bin/sh\nexit 1\n")
+    File.chmod(0o755, File.join(@plugin_dir, "run.sh"))
+    assert_raises(Maptool::Plugins::RunError) { Maptool::Plugins.run(@dir, "hello", [1, 2, 3, 4]) }
+  end
+
+  def test_run_raises_when_the_script_does_not_print_a_feature_collection
+    File.write(File.join(@plugin_dir, "run.sh"), "#!/bin/sh\necho 'not json'\n")
+    File.chmod(0o755, File.join(@plugin_dir, "run.sh"))
+    assert_raises(Maptool::Plugins::RunError) { Maptool::Plugins.run(@dir, "hello", [1, 2, 3, 4]) }
+  end
+end