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/ContentBlocks/ContentElements/video/templates/backend-preview.html

@@ -27,7 +27,7 @@
             <f:if condition="{data.header}">
                 <f:then>{data.header}</f:then>
                 <f:else>
-                    <em style="color:#c00;">⚠ Headline fehlt</em>
+                    <em style="color:#c00;">⚠ Headline missing</em>
                 </f:else>
             </f:if>
         </h3>
@@ -59,7 +59,7 @@
         </f:if>
 
         <f:if condition="!{data.youtube_code} && !{data.custom_video.0}">
-            <span class="vitec-badge vitec-badge--warning">⚠ Kein Video-Source</span>
+            <span class="vitec-badge vitec-badge--warning">⚠ No video source</span>
         </f:if>
         <f:if condition="{data.hide_controls}">
             <span class="vitec-badge">⏸ Controls hidden</span>