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/app/domains/products/service.py

@@ -107,9 +107,13 @@ def confirm_draft(conn: psycopg.Connection, storefront_id: int, account_id: int,
     catalog = repo.load_catalog(conn, storefront_id)
     diff_result = diff.compute_diff(catalog, products)
     if diff_result.fingerprint != row["fingerprint"]:
+        # Release the read snapshot; nothing written.
+        conn.rollback()
         raise PreviewStale()
     summary_counts = diff_result.summary
     if summary_counts["adds"] + summary_counts["updates"] == 0:
+        # Release the read snapshot; nothing written.
+        conn.rollback()
         raise NothingToApply()
     error_rows = [
         error.as_json()
@@ -179,12 +183,12 @@ def _apply_product_plan(conn: psycopg.Connection, storefront_id: int,
         else:
             repo.update_image(conn, image_plan.image_id, image_plan.changes)
 
-    # File order within the product, for variant adds without an explicit position.
-    file_positions = {id(v): i for i, v in enumerate(plan.canonical.variants, start=1)}
     for variant_plan in plan.variant_plans:
         if variant_plan.kind == "add":
-            fields = diff.resolved_fields(variant_plan.canonical.fields)
-            position = fields.get("position") or file_positions[id(variant_plan.canonical)]
+            fields = diff.resolved_variant_fields(variant_plan.canonical, variant_plan.file_order)
+            # diff time resolved any cleared position to file order; the
+            # file_order fallback covers an absent position column.
+            position = fields.get("position") or variant_plan.file_order
             url = fields.get("variant_image")
             image_id = image_ids[url] if url else None
             repo.insert_variant(