Add P4N_SESSION_COOKIE for p4n2gpx, since login alone can't work headless

Traced the plugin's "Login failed" error directly against
park4night.com: correct credentials still get rejected because the
login form is behind invisible reCAPTCHA, which only a real browser
ever satisfies. p4n2gpx.rb (outside this repo) now checks
P4N_SESSION_COOKIE first and uses it as-is, skipping the login POST
entirely — the cookie comes from an already-authenticated browser,
copied out by hand and refreshed when it expires. Wired the env var
through docker-compose.yaml and the deploy workflow's "Write .env"
step the same way as P4N_USER/P4N_PASSWORD.

.gitea/workflows/ci.yml

@@ -101,6 +101,7 @@ jobs:
           LIVETRACK_WEBHOOK_SECRET=${{ secrets.LIVETRACK_WEBHOOK_SECRET }}
           P4N_USER=${{ secrets.P4N_USER }}
           P4N_PASSWORD=${{ secrets.P4N_PASSWORD }}
+          P4N_SESSION_COOKIE=${{ secrets.P4N_SESSION_COOKIE }}
           EOF
           chmod 600 /srv/podman/maptool/.env