Wire up p4n2gpx as a private plugin

data/plugins/p4n2gpx/ (outside git, see .gitignore) now holds a
plugin.json + run.rb wrapper around ../p4n2gpx/ruby/p4n2gpx.rb, so it
runs through the same --south/--west/--north/--east-in,
GeoJSON-on-stdout-out contract as the built-in overpass-pois plugin.
run.rb converts the requested bbox to the center+radius p4n2gpx.rb
itself expects, then reuses lib/maptool/gpx.rb to turn its .gpx output
into GeoJSON.

p4n2gpx.rb needs faraday-cookie_jar, which wasn't in the Gemfile yet
(faraday itself already was, transitively via oauth2) — added and
locked. Also wires P4N_USER/P4N_PASSWORD through docker-compose.yaml
and the deploy workflow's "Write .env" step, the same optional-secret
pattern as LIVETRACK_WEBHOOK_SECRET, since the container has no
browser for p4n2gpx.rb's Chrome-cookie auth fallback.

docker-compose.yaml

@@ -31,6 +31,13 @@ services:
       # Optional: blank disables the LiveTrack webhook endpoint rather than
       # failing to start, since it's not needed for the app's core function.
       LIVETRACK_WEBHOOK_SECRET: ${LIVETRACK_WEBHOOK_SECRET:-}
+      # Optional, only read by data/plugins/p4n2gpx/'s own p4n2gpx.rb (not
+      # part of this repo — see docs/deployment.adoc) if that private
+      # plugin is present; blank just means it fails with its own "could
+      # not authenticate" error when run, same non-fatal-to-startup story
+      # as the other optional secrets here.
+      P4N_USER: ${P4N_USER:-}
+      P4N_PASSWORD: ${P4N_PASSWORD:-}
       # The kiwix service below, reached by its compose service name (both
       # are on the same default compose network) — port matches its own
       # --port=8080 in the command below. Not made conditional on the ZIM