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/FlexForms/Eventlist.xml

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<T3DataStructure>
+    <meta>
+        <langDisable>1</langDisable>
+    </meta>
+    <sheets>
+        <sDEF>
+            <ROOT>
+                <sheetTitle>Event List Settings</sheetTitle>
+                <type>array</type>
+                <el>
+                    <settings.daysinadvance>
+                        <label>Days in advance (0 = no limit)</label>
+                        <description>Only show upcoming events starting within the next N days, e.g. 60.</description>
+                        <config>
+                            <type>number</type>
+                            <default>0</default>
+                        </config>
+                    </settings.daysinadvance>
+
+                    <settings.showpast>
+                        <label>Include past events</label>
+                        <config>
+                            <type>check</type>
+                            <default>0</default>
+                        </config>
+                    </settings.showpast>
+
+                    <settings.limit>
+                        <label>Maximum number of events (0 = all)</label>
+                        <config>
+                            <type>number</type>
+                            <default>0</default>
+                        </config>
+                    </settings.limit>
+
+                    <settings.debug>
+                        <label>Allow Debug Output</label>
+                        <config>
+                            <type>check</type>
+                            <default>0</default>
+                        </config>
+                    </settings.debug>
+                </el>
+            </ROOT>
+        </sDEF>
+    </sheets>
+</T3DataStructure>