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/_frontend/.htaccess

@@ -9,8 +9,19 @@ RewriteRule ^ - [L]
 
 RewriteRule ^ index.html [L]
 
+# ── Cache headers ────────────────────────────────────────────────────────────
+
+# index.html: never cache — always fetch fresh so the browser picks up
+# the latest hashed JS/CSS filenames after a deploy.
 <Files "index.html">
-    Header set Cache-Control "no-cache, no-store, must-revalidate"
+    Header set Cache-Control "no-store, no-cache, must-revalidate"
     Header set Pragma "no-cache"
-    Header set Expires 0
-</Files>
+    Header set Expires "0"
+</Files>
+
+# Hashed assets (JS, CSS, fonts, images inside /assets/):
+# Vite appends a content hash to every filename, so these are immutable.
+# Cache them for 1 year — a new deploy produces new filenames automatically.
+<FilesMatch "\.(js|css|woff2?|ttf|eot|otf)$">
+    Header set Cache-Control "public, max-age=31536000, immutable"
+</FilesMatch>