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_helper.rb

@@ -14,6 +14,11 @@ require "json"
 ENV["MAPTOOL_DATA_DIR"] = Dir.mktmpdir("maptool-test-data")
 Minitest.after_run { FileUtils.remove_entry(ENV["MAPTOOL_DATA_DIR"]) }
 
+# A fixture plugin (test/fixtures/plugins/test-plugin), not the real
+# plugins/ — keeps /api/plugins route tests offline/deterministic instead
+# of actually running plugins/overpass-pois against the real Overpass API.
+ENV["MAPTOOL_PLUGINS_DIR"] = File.join(__dir__, "fixtures", "plugins")
+
 $LOAD_PATH.unshift(File.expand_path("../lib", __dir__))
 require_relative "../app"