Checkpoint: headless JSON architecture documented + cleanup (Stufe 1+2)

Restore point before Stufe 3 (central resolver-service refactoring).

Includes this session's work:
- Success Story (usecase) rebuild: new fields/tabs, UsecaseSerializer,
  thin List/Show renderers, MM relations, inline content elements.
- vitec_columns content block (two-column layout with per-item content)
  incl. unified header section; special-cased JSON resolution.
- Container per-column flex (items[].config align/justify) + gap on parent.
- Unified header section across CEs/plugins/containers; header fields in JSON.
- ISO-style architecture spec: Documentation/Headless-JSON-Architecture.md.
- Cleanup: removed local scratch + verified .bak backups.
- Fixes: B-6 (undefined thumbnail variable in Downloadcardcollection image
  resolver), B-7 (unsatisfiable legacy CType OR branch in Downloadcard/Datasheets).

Rollback: git reset --hard snapshot-2026-07-09-pre-stufe3

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/collateral-fallback.php

@@ -0,0 +1,140 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * Fallback resolver for deleted versioned files in fileadmin/downloads/Collateral.
+ *
+ * Expected requested filename format:
+ *   [fileprefix]__[filetype]__[number]-[letter].ext
+ *
+ * If a matching file no longer exists, this resolver redirects to the newest
+ * available version (highest number, then highest letter sequence).
+ */
+
+$requestedRaw = isset($_GET['requested']) ? (string)$_GET['requested'] : '';
+$requestedRaw = str_replace('\\', '/', $requestedRaw);
+$requestedFilename = basename(rawurldecode($requestedRaw));
+
+if ($requestedFilename === '' || $requestedFilename === '.' || $requestedFilename === '..') {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+if (!preg_match('/^(?P<prefix>.+?)__(?P<type>.+?)__(?P<number>\\d+)-(?P<letters>[A-Za-z]+)\\.(?P<ext>[A-Za-z0-9]+)$/', $requestedFilename, $parts)) {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+$prefix = (string)$parts['prefix'];
+$type = (string)$parts['type'];
+
+$baseDir = __DIR__ . '/fileadmin/downloads/Collateral';
+if (!is_dir($baseDir) || !is_readable($baseDir)) {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+$escapedPrefix = preg_quote($prefix, '/');
+$escapedType = preg_quote($type, '/');
+$pattern = '/^' . $escapedPrefix . '__' . $escapedType . '__(\\d+)-([A-Za-z]+)\\.([A-Za-z0-9]+)$/';
+
+$bestFilename = null;
+$bestNumber = -1;
+$bestLetterRank = -1;
+
+$entries = scandir($baseDir);
+if ($entries === false) {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+foreach ($entries as $entry) {
+    if (!is_string($entry) || $entry === '.' || $entry === '..') {
+        continue;
+    }
+
+    if (!preg_match($pattern, $entry, $matches)) {
+        continue;
+    }
+
+    $number = (int)$matches[1];
+    $letterRank = letterSequenceToRank((string)$matches[2]);
+
+    if ($number > $bestNumber || ($number === $bestNumber && $letterRank > $bestLetterRank)) {
+        $bestNumber = $number;
+        $bestLetterRank = $letterRank;
+        $bestFilename = $entry;
+    }
+}
+
+if ($bestFilename === null) {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+$bestPath = $baseDir . '/' . $bestFilename;
+if (!is_file($bestPath) || !is_readable($bestPath)) {
+    http_response_code(404);
+    header('Content-Type: text/plain; charset=utf-8');
+    echo 'Not found';
+    exit;
+}
+
+$clientIp = isset($_SERVER['REMOTE_ADDR']) ? (string)$_SERVER['REMOTE_ADDR'] : '-';
+$userAgent = isset($_SERVER['HTTP_USER_AGENT']) ? (string)$_SERVER['HTTP_USER_AGENT'] : '-';
+logFallbackRecovery($requestedFilename, $bestFilename, $clientIp, $userAgent);
+
+$target = '/fileadmin/downloads/Collateral/' . rawurlencode($bestFilename);
+header('Cache-Control: no-store, no-cache, must-revalidate, max-age=0');
+header('Pragma: no-cache');
+header('Location: ' . $target, true, 302);
+exit;
+
+function letterSequenceToRank(string $letters): int
+{
+    $rank = 0;
+    $letters = strtoupper($letters);
+    $length = strlen($letters);
+
+    for ($i = 0; $i < $length; $i++) {
+        $char = ord($letters[$i]);
+        if ($char < 65 || $char > 90) {
+            continue;
+        }
+
+        $rank = ($rank * 26) + ($char - 64);
+    }
+
+    return $rank;
+}
+
+function logFallbackRecovery(string $requested, string $resolved, string $clientIp, string $userAgent): void
+{
+    $logDir = dirname(__DIR__) . '/var/log';
+    if (!is_dir($logDir)) {
+        return;
+    }
+
+    $timestamp = gmdate('c');
+    $line = sprintf(
+        "%s collateral-fallback requested=\"%s\" resolved=\"%s\" ip=%s ua=\"%s\"\n",
+        $timestamp,
+        str_replace('"', '\\"', $requested),
+        str_replace('"', '\\"', $resolved),
+        $clientIp,
+        str_replace('"', '\\"', $userAgent)
+    );
+
+    @file_put_contents($logDir . '/collateral-fallback.log', $line, FILE_APPEND | LOCK_EX);
+}