Support a persistent, private plugins directory alongside the built-in ones

plugins/ ships baked into the image and gets wiped/replaced by the
checkout sync on every deploy (docs/deployment.adoc), so a private
plugin dropped there — the exact use case the plugin contract was
designed for — didn't survive a redeploy the way uploaded GPX/kiwix
data under data/ does.

Plugins.list/run now accept either one directory or an array, and
app.rb checks the built-in plugins/ plus data/plugins/ (already
covered by the existing ./data bind mount, no new volume needed).

docker-compose.yaml

@@ -45,7 +45,9 @@ services:
       # Uploaded GPX files and the offline PMTiles extract — kept outside
       # the image so a rebuild/redeploy doesn't wipe them (see
       # docs/development.adoc for how the deploy job treats this
-      # directory).
+      # directory). A private plugin (app.rb's PLUGINS_DIRS) also belongs
+      # under here, at data/plugins/<name>/ — nothing further to mount,
+      # this one bind already covers the whole data/ tree.
       - ./data:/app/data
 
   # Fakes a GNSS fix (a loop around Bratislava, see