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

CHANGELOG.md

@@ -22,7 +22,7 @@ version tags are cut in this repo — POC stage, `main` is the release line).
   (`extract_do_fields` tesseract|paddle|none).
 - Operator DO panel + OCR toggle; monitoring 3-way mode switch + model mode +
   OCR/require-plate controls; history/export plate, DO, expected, net columns.
-- `docs/do-erd.md` (Mermaid ERD); tests `tests/test_do_batch.py`.
+- `ERD.md` (Mermaid ERD, repo root; superseded `docs/do-erd.md`); tests `tests/test_do_batch.py`.
 
 ### Changed
 - Default batch mode seed **`auto`** (was hard-coded `manual` when