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

README.md

@@ -13,8 +13,13 @@ src/yolo/defaults.py              Project defaults
 src/yolo/engine.py                Train, validate, predict, and export
 src/yolo/checkpoints.py           Checkpoint discovery and resume support
 src/yolo/reporting.py             Training result reporting
-backend/                          FastAPI inference API and alerting
-frontend/                         Browser management platform (no build step)
+backend/main.py                   FastAPI app, detect/events/robots endpoints
+backend/alerting.py               Consecutive-frame confirmation, WeChat/Feishu delivery
+backend/events.py                 Alert event store (SQLite-backed)
+backend/storage.py                SQLite database, delivery stats, UTC timestamps
+frontend/main.js                  Browser entry, view switching, boot sequence
+frontend/modules/                 Shared helpers (dom, api, ui, charts, state)
+frontend/views/                   Per-view rendering (dashboard, inspection, events, robots)
 ```
 
 ## Setup
@@ -100,3 +105,7 @@ Detection requests without a `session_id` share the fixed `single-image` session
 - `data/Smoke-Fire-Detection-YOLO/` — main smoke/fire dataset (train/val/test).
 - `data/fire-dataset/` — incremental hard-negative collection (395 train / 87 val images, Pascal VOC XML plus YOLO labels); not yet wired to a training config.
 - `runs/audit/` — label audit tooling: `scan_missing_labels.py` finds unlabeled images that the current model detects, `render_missing_label_candidates.py` renders them for review.
+
+## Roadmap
+
+The development direction (hard-negative fine-tuning, YOLO26n comparison, real-time push, multi-video concurrency, data flywheel, and more) is tracked in [`docs/roadmap.md`](docs/roadmap.md).