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/cta-banner/templates/backend-preview.html

@@ -6,7 +6,7 @@
 
 <div class="vitec-preview vitec-preview--{data.background_variant}">
 
-    <f:comment>Thumbnail (Background Image oder Farb-Placeholder)</f:comment>
+    <f:comment>Thumbnail (background image or colour placeholder)</f:comment>
     <f:if condition="{data.background_image.0}">
         <f:then>
             <f:image image="{data.background_image.0}"
@@ -37,7 +37,7 @@
             <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>
@@ -75,7 +75,7 @@
             </f:switch>
         </span>
 
-        <f:comment>Layout Variant Badge mit Icon</f:comment>
+        <f:comment>Layout variant badge with icon</f:comment>
         <span class="vitec-badge">
             <f:switch expression="{data.layout_variant}">
                 <f:case value="centered">◎ Centered</f:case>
@@ -85,9 +85,9 @@
             </f:switch>
         </span>
 
-        <f:comment>Warnung wenn Primary-CTA-Link fehlt</f:comment>
+        <f:comment>Warning when primary CTA link is missing</f:comment>
         <f:if condition="{data.primary_cta.url} == ''">
-            <span class="vitec-badge vitec-badge--warning">⚠ Kein Primary-Link</span>
+            <span class="vitec-badge vitec-badge--warning">⚠ Primary link missing</span>
         </f:if>
     </div>