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>

public/.htaccess

@@ -318,6 +318,11 @@ AddDefaultCharset utf-8
 	RewriteCond %{SCRIPT_FILENAME} -f
 	RewriteRule (?:^|/)\. - [F]
 
+	# Redirect missing hardcoded Collateral file URLs to a resolver that
+	# chooses the newest available version by filename convention.
+	RewriteCond %{REQUEST_FILENAME} !-f
+	RewriteRule ^fileadmin/downloads/Collateral/(.+)$ %{ENV:CWD}collateral-fallback.php?requested=$1 [QSA,L,NC]
+
 	# Stop rewrite processing, if we are in any known directory
 	# NOTE: Add your additional local storages here
 	RewriteRule ^(?:fileadmin/|typo3conf/|typo3temp/|uploads/) - [L]
@@ -392,7 +397,7 @@ FileETag None
 
 # Add your own rules here.
 
-AuthUserFile "/usr/www/users/vitecevo/live/public/.htpasswd"
-AuthName "DEV"
-AuthType Basic
-require valid-user
+#AuthUserFile "/usr/www/users/vitecevo/live/public/.htpasswd"
+#AuthName "DEV"
+#AuthType Basic
+#require valid-user