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>

.gitea/workflows/deploy.yml

@@ -86,7 +86,7 @@ jobs:
       # the generator's geometry. The provider suites use a fake transport: they
       # prove the documented contract, not the integration.
       - name: Print-file geometry and provider adapters
-        run: $COMPOSE exec -T api python -m unittest tests.test_printfile tests.test_mercadopago tests.test_tiny tests.test_jadlog tests.test_quote_review tests.test_large_files -v
+        run: $COMPOSE exec -T api python -m unittest tests.test_printfile tests.test_mercadopago tests.test_tiny tests.test_jadlog tests.test_quote_review tests.test_large_files tests.test_grade_check -v
 
       - name: Runtime and retention regressions
         run: |