fix(products): cleared Variant Position resolves to file order, not NULL

A blank Variant Position cell previewed position->null and aborted the
confirm transaction (variant.position is NOT NULL). Resolve the clear to
the variant's file order at diff time so preview and apply stay in
lockstep; carry file_order on VariantPlan instead of an identity-keyed
map; release the read snapshot before PreviewStale/NothingToApply.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/tests/test_products_service.py

@@ -150,6 +150,25 @@ def test_confirm_update_changes_only_diffed_fields(migrated_conn, merchant):
     assert migrated_conn.execute("SELECT count(*) FROM product").fetchone()[0] == 2  # no dupes (BUC-3)
 
 
+def test_confirm_blank_position_cell_round_trips(migrated_conn, merchant):
+    # A present-but-empty Variant Position cell resolves to file order at diff
+    # time — never SET position = NULL (which would abort the confirm on the
+    # NOT NULL constraint).
+    d1 = _validate(migrated_conn, merchant)
+    products.confirm_draft(migrated_conn, merchant["storefront_id"], merchant["account_id"], d1["id"])
+    blank_position_csv = (
+        b"Handle,Title,Vendor,Variant Price,Variant Position\n"
+        b"moon-mug,Moon Mug,Acme,21.00,\n"
+        b"star-tee,Star Tee,Acme,24.00,\n"
+    )
+    d2 = _validate(migrated_conn, merchant, blank_position_csv)
+    products.confirm_draft(migrated_conn, merchant["storefront_id"], merchant["account_id"], d2["id"])
+    price = migrated_conn.execute(
+        "SELECT v.price FROM variant v JOIN product p ON p.id = v.product_id WHERE p.handle='moon-mug'"
+    ).fetchone()[0]
+    assert str(price) == "21.00"
+
+
 def test_confirm_mixed_applies_valid_records_errors(migrated_conn, merchant):
     draft = _validate(migrated_conn, merchant, MIXED_CSV)
     run_id = products.confirm_draft(