doc: log when DB is persisted on shutdown

Author: metacosm

backend/src/main/java/net/laprun/sustainability/power/persistence/SQLiteFilePersister.java

@@ -46,6 +46,7 @@ void scheduled() {
 
     // Execute a backup during shutdown
     public void onShutdown(@Observes ShutdownEvent event) {
+        Log.info("Persisting database on shutdown");
         backup();
     }