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/Resources/Private/Templates/Downloadcardcollection/Backendpreview.html

@@ -13,15 +13,15 @@
             <f:if condition="{data.header}">
                 <f:then>{data.header}</f:then>
                 <f:else>
-                    <em style="color:#888;">Download Card Collection (kein Header)</em>
+                    <em style="color:#888;">Download Card Collection (no header)</em>
                 </f:else>
             </f:if>
         </h3>
 
         <div class="vitec-preview__body-text">
             <small>
-                Multi-Download-Auswahl mit optionalem Header-Image.
-                Konfiguration im FlexForm.
+                Multi-download selection with optional header image.
+                Configured via FlexForm.
             </small>
         </div>
     </div>