feat(backend): add public pound-qr endpoint by inventory id

Expose a read-only payload for the shared weighbridge QR card without auth.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/tests/test_public_db_pound_qr.py

@@ -0,0 +1,50 @@
+"""公开磅单二维码只读接口。"""
+from datetime import date
+from types import SimpleNamespace
+
+from fastapi.testclient import TestClient
+
+from app.main import app
+from app.services.db.pound_qr_public import (
+    build_pound_qr_label,
+    build_public_pound_qr_read,
+)
+
+client = TestClient(app)
+
+
+def test_build_pound_qr_label():
+    assert build_pound_qr_label("出库", "合肥德泽") == "出库 · 合肥德泽"
+    assert build_pound_qr_label("入库", "") == "入库"
+    assert build_pound_qr_label("", "安庆山峡") == "安庆山峡"
+
+
+def test_build_public_pound_qr_read_maps_fields():
+    row = SimpleNamespace(
+        id="inv-1",
+        date=date(2026, 9, 12),
+        business_type="出库",
+        vehicle_no="皖AM8516",
+        partner_company="合肥德泽化工科技有限公司",
+        product="30% 工业硫酸",
+        specification="30%",
+        quantity=10.0,
+        remark="注意安全",
+    )
+    read = build_public_pound_qr_read(row, quantity_unit="吨")
+    dump = read.model_dump(by_alias=True)
+    assert dump["id"] == "inv-1"
+    assert dump["date"] == "2026-09-12"
+    assert dump["businessType"] == "出库"
+    assert dump["vehicleNo"] == "皖AM8516"
+    assert dump["partnerCompany"] == "合肥德泽化工科技有限公司"
+    assert dump["product"] == "30% 工业硫酸"
+    assert dump["specification"] == "30%"
+    assert dump["quantity"] == 10.0
+    assert dump["quantityUnit"] == "吨"
+    assert dump["remarks"] == "注意安全"
+
+
+def test_pound_qr_missing_returns_404():
+    resp = client.get("/api/public/db/inventory/__missing_pound_qr_id__/pound-qr")
+    assert resp.status_code == 404