Content checks: flag placeholder images instead of counting them as done

Files below fileadmin/placeholders/ (or named "...placeholder...") are grey
dummies. PageCheckService and ProductCheckService now report them:

- image cells carry `placeholder` = number of placeholder files (only when > 0);
  `ok` is unchanged (something is attached)
- summary.filled counts only cells without placeholders, new summary.placeholder
  counts the cells that have one
- detail images carry `placeholder: true|false`
- cards: `placeholder` = cards whose icon/image is a placeholder; record: the
  record image

Additive change to the JSON contract; frontend rendering follows (ticket).
Requested by Harry 2026-11-13.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

migrations/pagecheck_placeholder_20261113.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+# Placeholder flag for the content checks (2026-11-13): permissions, syntax check,
+# cache flush and SSR purge of the three check pages; "commit" as first argument
+# commits the two services afterwards.
+set -e
+cd /usr/www/users/vitecevo/live
+SRV=packages/vitec/Classes/Service
+chmod 644 $SRV/PageCheckService.php $SRV/ProductCheckService.php migrations/pagecheck_placeholder_20261113.sh
+php -l $SRV/PageCheckService.php
+php -l $SRV/ProductCheckService.php
+if [ "$1" = "commit" ]; then
+  git add $SRV/PageCheckService.php $SRV/ProductCheckService.php migrations/pagecheck_placeholder_20261113.sh
+  git commit -q -F migrations/pagecheck_placeholder_20261113.msg
+  git log --oneline -1
+  exit 0
+fi
+vendor/bin/typo3 cache:flush
+vendor/bin/typo3 vitec:ssr-purge /markets/market-check /solutions/solution-check /products/product-check
+vendor/bin/typo3 vitec:ssr-warm /markets/market-check /solutions/solution-check /products/product-check