(ops) Expand native install self checks and recovery docs

Extend the runtime self check with native install diagnostics for the
environment file, service user, backup root ownership, and SQLite database
path. Export install metadata from the systemd units and pobsync-manage wrapper
so custom env files and service users are visible to Django checks.

Document restart, journal log inspection, and rollback steps in the README so
production updates have a clear recovery path.

scripts/install-systemd

@@ -463,6 +463,9 @@ POBSYNC_BACKUP_ROOT=$BACKUP_ROOT
 POBSYNC_TIME_ZONE=$TIME_ZONE
 POBSYNC_SQLITE_PATH=/var/lib/pobsync/pobsync.sqlite3
 POBSYNC_STATIC_ROOT=/var/lib/pobsync/static
+POBSYNC_ENV_FILE=$ENV_FILE
+POBSYNC_SERVICE_USER=$SERVICE_USER
+POBSYNC_SERVICE_GROUP=$SERVICE_GROUP
 
 POBSYNC_WEB_BIND=$WEB_BIND
 POBSYNC_GUNICORN_WORKERS=2
@@ -509,6 +512,8 @@ install_manage_wrapper() {
         -e "s|@POBSYNC_APP_DIR@|$APP_DIR|g" \
         -e "s|@POBSYNC_VENV_DIR@|$VENV_DIR|g" \
         -e "s|@POBSYNC_ENV_FILE@|$ENV_FILE|g" \
+        -e "s|@POBSYNC_USER@|$SERVICE_USER|g" \
+        -e "s|@POBSYNC_GROUP@|$SERVICE_GROUP|g" \
         "$APP_DIR/deploy/bin/pobsync-manage" > /usr/local/bin/pobsync-manage
     chmod 0755 /usr/local/bin/pobsync-manage
 }