feat: the server checks the grade against the files before approving

The price depends on the grade, which the browser worked out and the API
took on trust. Before a cart is approved at checkout the API now recomputes
it from the uploaded files by the Site's own rules: the pixel size in a
PNG, JPG or WebP header across the printed width (rotation included), and
the area-weighted DPI of the images placed in a PDF of up to 150 MB, 300
for vectors. Sheets take the worst grade, artworks the average. A claim more
than 2 points above the file's grade, or a discount on a file the server
cannot grade, waits for an operator, with the reason on the Kanban.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -627,6 +627,13 @@ the site already did.
 generator, with the browser as preview only. This is the single largest gap between
 what was promised in the meeting and what exists.
 
+**2026-09-30:** the grade (the resolution discount) is no longer taken on trust.
+Before an automatic approval the API recomputes it from the uploaded files by the
+Site's rules (`app/grade_check.py`): image headers for PNG/JPG/WebP, the placed
+images of a PDF up to 150 MB. A claim more than 2 points above the file's grade,
+or a discount on a file the server cannot grade, waits for review with the reason
+on the Kanban. The metres (the packing) are still the browser's.
+
 ### `[~]` 3.3 — The 5 GB problem is unsolved `(F19)`
 
 Transport accepts 5 GiB; `SCAN_MAX_BYTES` / ClamAV `StreamMaxLength` release only