Add Garmin LiveTrack session tracking

A Gmail-watching Apps Script (documented in docs/deployment.adoc) posts
the session ID from each LiveTrack email to a new webhook, which stores
it in data/livetrack.json; a new sidebar panel lists what's been
received.

docker-compose.yaml

@@ -17,6 +17,9 @@ services:
       GITEA_OAUTH_CLIENT_ID: ${GITEA_OAUTH_CLIENT_ID:?GITEA_OAUTH_CLIENT_ID is required, see docs/deployment.adoc}
       GITEA_OAUTH_CLIENT_SECRET: ${GITEA_OAUTH_CLIENT_SECRET:?GITEA_OAUTH_CLIENT_SECRET is required, see docs/deployment.adoc}
       MAPTOOL_ALLOWED_GITEA_USER: ${MAPTOOL_ALLOWED_GITEA_USER:?MAPTOOL_ALLOWED_GITEA_USER is required, see docs/deployment.adoc}
+      # 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:-}
     volumes:
       # Uploaded GPX files and the offline PMTiles extract — kept outside
       # the image so a rebuild/redeploy doesn't wipe them (see