feat: reorder inventory images atomically

docs/API.md

@@ -197,3 +197,13 @@ Seller contact removal uses the same explicit-empty-string convention:
 whitespace-only strings clear the contact. Omitted contact fields retain their
 saved values. `null` is not a settings input. Public listing/product detail DTOs
 return `null` for empty or absent contacts, so no contact link is rendered.
+
+`POST /api/v1/admin/images/:id/move` accepts exactly `{ "direction": "up" }`
+or `{ "direction": "down" }`. Authentication and session CSRF validation are
+required; ownership comes from the authenticated Seller. The server selects the
+adjacent image within the same physical Inventory Item and commits all resulting
+positions atomically. At the first/last position the corresponding move is a
+successful no-op. Missing/unowned images return `404 IMAGE_NOT_FOUND`; invalid
+bodies return `400 VALIDATION_ERROR`. The response uses the existing admin image
+DTO. The legacy absolute `/order` endpoint remains available, but UI moves use
+this new operation exclusively.