Use default database file if none is specified

backend/main.go

@@ -558,5 +558,9 @@ func main() {
 		Datasource: os.Getenv("DB_PATH"),
 		Addr:       ":8080",
 	}
+	if config.Datasource == "" {
+		config.Datasource = "allowance_planner.db3"
+		log.Printf("Warning: No DB_PATH set, using default of %s", config.Datasource)
+	}
 	start(context.Background(), &config)
 }