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/page.tsconfig

@@ -80,6 +80,14 @@ mod {
                     CType = vitec_datasheets
                 }
             }
+            eventlist {
+                iconIdentifier = vitec-plugin-eventlist
+                title = VITEC Event List
+                description = Upcoming VITEC events (trade shows) with date, venue, booth and links
+                tt_content_defValues {
+                    CType = vitec_eventlist
+                }
+            }
             simplecard {
                 iconIdentifier = vitec-plugin-simplecard
                 title = Simple Card
@@ -97,6 +105,13 @@ mod {
     }
 }
 
+tx_news.templateLayouts {
+    1 = Compact List
+    2 = Featured Story
+    3 = Editorial Grid
+    4 = Detail Spotlight
+}
+
 ############################################################
 # Backend Previews für klassische Plugins (list_type)
 ############################################################