Add serve-webapp.sh to manage the webapp stack on the lightweight machine

Single command to start the interactive map container in the background
(auto-restart after reboot), with stop/restart/status/sync/logs/update
subcommands. Configuration lives in webapp.env (template provided,
git-ignored): remote generation URL, shared token, rsync cache command,
periodic sync and allowed network. The SSH key is mounted automatically
for rsync, port conflicts are detected before launch, and update pulls the
repo, rebuilds the image and restarts.

Deploy doc updated: the script is now the recommended install path on the
Raspberry Pi, alongside run.sh (foreground) and docker compose.

.gitignore

@@ -42,6 +42,7 @@ htmlcov/
 # Fichiers de configuration locaux
 .env
 .env.local
+webapp.env
 
 # Données et artefacts internes (jamais dans le dépôt)
 data/