fix: use packed artwork model for standard image uploads

CONTEXT.md

@@ -153,6 +153,14 @@ The browser may display the calculation, but production payment creation must
 recalculate and validate all price, quantity, freight, and discount values on
 the server. Client-provided totals are never authoritative.
 
+Normal JPG/PNG uploads are individual artwork by default, including when uploaded
+from the by-metre panel. They use the existing loose-artwork editor, printed
+width, quantity, and live packing canvas; packed height determines metres.
+Customers uploading an already assembled film-width image must explicitly select
+the ready-sheet option before uploading. PDF and manual-format ready sheets keep
+their existing workflow. These models retain their separate approved price tiers;
+image dimensions alone must never classify a normal artwork as a finished sheet.
+
 ## Freight
 
 The original visual freight flow in `dtf-site.html` was a stub with only pickup