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/fixtures/plugins/test-plugin/plugin.json

@@ -0,0 +1,4 @@
+{
+  "label": "Test plugin",
+  "command": "./run.sh"
+}

test/fixtures/plugins/test-plugin/run.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+# A fixture plugin for test_app.rb's /api/plugins route tests — no
+# network, just echoes a single fixed point back as GeoJSON so those tests
+# stay deterministic and offline like the rest of the suite.
+echo '{"type":"FeatureCollection","features":[{"type":"Feature","properties":{"name":"Test POI"},"geometry":{"type":"Point","coordinates":[17.0,48.0]}}]}'