Expire stored source photos after 7 days, keeping grades forever

Storing originals for Regrade grew the database ~16MB per grade at the
upload cap. Photos now expire on a configurable window (default 7 days,
counted from last grade so regrading resets it) and the file is VACUUMed so
the space is actually returned — clearing the column alone only moves pages
to the freelist, which would have made the whole feature a no-op on disk.
Grades, thumbnails and measurements are never pruned; a pruned card's
Regrade falls back to the existing re-pick path, now with a banner saying
why.

docker-compose.yml

@@ -35,3 +35,10 @@ services:
       # This app is reverse-proxied at hippofam.com/cards, not the domain
       # root — see app.py's BASE_PATH handling.
       - CARD_GRADER_BASE_PATH=/cards
+      # How long a grade keeps the original photo(s) that produced it, so
+      # Regrade can re-run without asking for them again. Grades, their
+      # thumbnails and every measurement are kept FOREVER regardless — only
+      # the photos expire, since they're what makes the database grow (up
+      # to ~16MB per grade at the upload cap). Past the window, Regrade
+      # falls back to asking for the photo. 0 disables pruning.
+      - CARD_GRADER_IMAGE_RETENTION_DAYS=7