deploy.sh: remove the images a deploy replaced

Each deploy left a 321 MB app image dangling on the NVMe (13 after a day
of deploys).

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

deploy/deploy.sh

@@ -48,5 +48,7 @@ docker compose stop edge
 docker compose up -d --no-deps web coordinator
 docker compose up -d --no-deps edge
 echo "$SHA $(date -Is)" >> "$ROOT/deployed.log"
+# The images this replaced (321 MB each; the root filesystem is the NVMe, with a 10 GB floor).
+docker image prune -f --filter label=com.docker.compose.project=usenet-rebuild >/dev/null
 docker compose ps --format '{{.Service}} {{.Status}}' web coordinator edge
 echo "== deployed $SHA"