docs: sync README layout with new backend/frontend modules, add roadmap

- Expand project layout tree with backend/{storage,events,alerting,main}.py
  and the modular frontend (main.js + modules/ + views/)
- Add docs/roadmap.md with the three-phase development direction and link
  it from the README
- List YOLO_DB_PATH in the settings view environment reference

frontend/index.html

@@ -199,6 +199,7 @@ YOLO_DEVICE=0
 YOLO_IMGSZ=768
 YOLO_CONF=0.40
 YOLO_IOU=0.45
+YOLO_DB_PATH=data/app.db
 ALERT_CONFIRM_FRAMES=3
 ALERT_COOLDOWN_SECONDS=60</code></pre></article>
           </section>