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/Icons.php

@@ -32,6 +32,10 @@ return [
         'provider' => SvgIconProvider::class,
         'source' => 'EXT:vitec/Resources/Public/Icons/vitec-plugin-usecaselist.svg',
     ],
+    'vitec-plugin-eventlist' => [
+        'provider' => SvgIconProvider::class,
+        'source' => 'EXT:vitec/Resources/Public/Icons/vitec-plugin-eventlist.svg',
+    ],
     'vitec-plugin-downloadcard' => [
         'provider' => SvgIconProvider::class,
         'source' => 'EXT:vitec/Resources/Public/Icons/vitec-plugin-downloadcard.svg',