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>

packages/vitec/Configuration/TCA/Overrides/tt_content.php

@@ -34,6 +34,7 @@ use TYPO3\CMS\Extbase\Utility\ExtensionUtility;
     $registerPluginWithFlexForm('Downloadcard', 'Download Card', 'FILE:EXT:vitec/Configuration/FlexForms/Downloadcard.xml', 'vitec-plugin-downloadcard');
     $registerPluginWithFlexForm('Downloadcardcollection', 'Download Card Collection', 'FILE:EXT:vitec/Configuration/FlexForms/Downloadcardcollection.xml', 'vitec-plugin-downloadcardcollection');
     $registerPluginWithFlexForm('Datasheets', 'Datasheets', 'FILE:EXT:vitec/Configuration/FlexForms/Datasheets.xml');
+    $registerPluginWithFlexForm('Eventlist', 'Event List', 'FILE:EXT:vitec/Configuration/FlexForms/Eventlist.xml', 'vitec-plugin-eventlist');
 
     // Change frame_class to allow multiple selections.
     $GLOBALS['TCA']['tt_content']['columns']['frame_class']['config']['renderType'] = 'selectCheckBox';
@@ -50,4 +51,9 @@ use TYPO3\CMS\Extbase\Utility\ExtensionUtility;
             ['label' => 'Vitec: Highlight Box', 'value' => 'vitec-highlight'],
         ]
     );
+
+    // Inline parent pointer: content elements owned by a Success Story record.
+    $GLOBALS['TCA']['tt_content']['columns']['tx_vitec_usecase_content'] = [
+        'config' => ['type' => 'passthrough'],
+    ];
 })();