docs: root ERD.md (SQLite ERD, Mermaid), merge docs/do-erd.md

AGENTS.md

@@ -40,7 +40,7 @@ Indonesian (`karung`=sack, `truk`=truck); YOLO class names are English
   office `:5721` only (403 on `:5000`); **`ocr_engine` POST both ports** (synced
   via `GET /api/do/settings`). Discard batch only if **both** `net_sack==0` and
   `net_box==0`. DO helpers: `src/do_batch.py`, OCR: `src/do_ocr.py`;
-  ERD: `docs/do-erd.md`.
+  ERD: `ERD.md` (repo root).
 - **Counting filters by class name, not ID**: `SackDetector`/`BoxDetector`/
   `TruckDetector` filter via `BaseDetector(class_filter)` (`src/detection.py`);
   tracker keeps `("sack", "truck", "box")` (`src/tracking.py`); counting uses