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/diff.py

@@ -11,7 +11,9 @@ fingerprint over the records detect catalog drift between preview and confirm.
 Deliberately DB-free: the catalog snapshot dataclasses are defined here and the
 repo layer builds them. Only fields present in the file's canonical fields{}
 participate in a comparison — an absent column is untouched, never a change
-(§6.5.1); a present-but-empty cell resolves to the field's CLEAR_DEFAULTS entry.
+(§6.5.1); a present-but-empty cell resolves to the field's CLEAR_DEFAULTS entry
+— except a variant's position, whose default is the variant's 1-based file
+order within its product ("defaults to file order"), resolved here at diff time.
 Catalog variants/images absent from the file are likewise untouched (INV-10);
 file variants match catalog variants by their option-value combination (INV-13).
 """
@@ -69,6 +71,8 @@ class VariantPlan:
     kind: str  # "add" | "update"
     canonical: CanonicalVariant
     catalog_id: int | None  # None for add
+    # 1-based index within the product's file variants — the position default.
+    file_order: int = 0
     # field -> resolved after-value (update only); adds resolve from canonical.fields
     changes: dict[str, object] = field(default_factory=dict)
 
@@ -145,7 +149,10 @@ def _add_plan(product: CanonicalProduct) -> ProductPlan:
         kind="add",
         canonical=product,
         catalog=None,
-        variant_plans=[VariantPlan(kind="add", canonical=v, catalog_id=None) for v in product.variants],
+        variant_plans=[
+            VariantPlan(kind="add", canonical=v, catalog_id=None, file_order=order)
+            for order, v in enumerate(product.variants, start=1)
+        ],
         image_plans=[
             ImagePlan(kind="add", source_url=i.source_url, position=i.position,
                       alt_text=i.alt_text, image_id=None)
@@ -164,7 +171,8 @@ def _add_detail(plan: ProductPlan) -> dict:
         "set": {f: _json_safe(v) for f, v in set_fields.items()},
         "option_names": list(plan.canonical.option_names),
         "variants": [
-            {"options": list(vp.canonical.options), "set": _resolved_variant_fields(vp.canonical)}
+            {"options": list(vp.canonical.options),
+             "set": _resolved_variant_fields(vp.canonical, vp.file_order)}
             for vp in plan.variant_plans
         ],
         "images": [
@@ -200,17 +208,20 @@ def _update_plan(product: CanonicalProduct, current: CatalogProduct) -> tuple[Pr
     variant_plans: list[VariantPlan] = []
     variant_entries: list[dict] = []
     by_options = {v.options: v for v in current.variants}
-    for variant in product.variants:
+    for file_order, variant in enumerate(product.variants, start=1):
         match = by_options.get(variant.options)
         if match is None:
-            variant_plans.append(VariantPlan(kind="add", canonical=variant, catalog_id=None))
+            variant_plans.append(
+                VariantPlan(kind="add", canonical=variant, catalog_id=None, file_order=file_order)
+            )
             variant_entries.append(
-                {"options": list(variant.options), "kind": "add", "set": _resolved_variant_fields(variant)}
+                {"options": list(variant.options), "kind": "add",
+                 "set": _resolved_variant_fields(variant, file_order)}
             )
             continue
         variant_changes: dict[str, object] = {}
         variant_change_entries: list[dict] = []
-        for f, resolved in resolved_fields(variant.fields).items():
+        for f, resolved in resolved_variant_fields(variant, file_order).items():
             # position lives on the catalog variant as an attribute, not in
             # fields{} — compare it explicitly (as images do for theirs).
             before = match.position if f == "position" else match.fields.get(f)
@@ -219,7 +230,8 @@ def _update_plan(product: CanonicalProduct, current: CatalogProduct) -> tuple[Pr
                 variant_change_entries.append(_change(f, before, resolved))
         if variant_changes:
             variant_plans.append(
-                VariantPlan(kind="update", canonical=variant, catalog_id=match.id, changes=variant_changes)
+                VariantPlan(kind="update", canonical=variant, catalog_id=match.id,
+                            file_order=file_order, changes=variant_changes)
             )
             variant_entries.append(
                 {"options": list(variant.options), "kind": "update", "changes": variant_change_entries}
@@ -284,8 +296,18 @@ def resolved_product_fields(product: CanonicalProduct) -> dict[str, object]:
     }
 
 
-def _resolved_variant_fields(variant: CanonicalVariant) -> dict[str, object]:
-    return {f: _json_safe(v) for f, v in resolved_fields(variant.fields).items()}
+def resolved_variant_fields(variant: CanonicalVariant, file_order: int) -> dict[str, object]:
+    """File-present variant fields with clears resolved. A cleared position has
+    no CLEAR_DEFAULTS entry — it resets to the variant's 1-based file order
+    within its product ("defaults to file order"), never to NULL."""
+    resolved = resolved_fields(variant.fields)
+    if "position" in resolved and resolved["position"] is None:
+        resolved["position"] = file_order
+    return resolved
+
+
+def _resolved_variant_fields(variant: CanonicalVariant, file_order: int) -> dict[str, object]:
+    return {f: _json_safe(v) for f, v in resolved_variant_fields(variant, file_order).items()}
 
 
 def _change(field_name: str, before: object, after: object) -> dict:

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(