fix(pl): keep multi-brand site feedback when plan product row is stale

Persist submitted brand qtys even if plan brand_mode lookup fails; resolve soft-deleted plan products for brand metadata instead of substituting another product; stop seeding all actual qty onto the first brand option.

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

backend/tests/test_pl_feedback_brand_qty.py

@@ -73,3 +73,56 @@ def test_require_calls_validate_when_positive_rows(monkeypatch):
     assert called["brand_mode"] == "any"
     assert called["actual_qty"] == Decimal("10")
     assert called["rows"] == rows
+
+
+def test_persist_io_item_brand_qtys_writes_when_brand_mode_missing():
+    """计划行 brand_mode 丢失时，客户端提交的多品牌明细仍应落库（不能只软删不写）。"""
+    from app.services.pl_feedback_brand_qty import persist_io_item_brand_qtys
+
+    class _Row:
+        def __init__(self):
+            self.del_flag = 0
+            self.update_by = None
+
+    existing = [_Row()]
+    added = []
+
+    class _Query:
+        def filter(self, *args, **kwargs):
+            return self
+
+        def all(self):
+            return existing
+
+    class _Db:
+        def query(self, model):
+            return _Query()
+
+        def add(self, obj):
+            added.append(obj)
+
+    class _Model:
+        order_item_id = object()
+        tenant_no = object()
+        del_flag = object()
+
+        def __init__(self, **kwargs):
+            self.__dict__.update(kwargs)
+
+    persist_io_item_brand_qtys(
+        _Db(),
+        model=_Model,
+        tenant_no="t1",
+        order_id="o1",
+        order_item_id="i1",
+        plan_product_id="p1",
+        brand_mode=None,
+        rows=[
+            {"brandId": "b1", "brandName": "君正", "qty": Decimal("20")},
+            {"brandId": "b2", "brandName": "红四方", "qty": Decimal("30")},
+        ],
+        user_id="u1",
+    )
+    assert existing[0].del_flag == 1
+    assert len(added) == 2
+    assert {a.brand_id for a in added} == {"b1", "b2"}