Media-Previews: _preview.jpg bei Upload, alle Listenansichten — SW by-v437, APP_VER 416

- media_utils: generate_preview() (Pillow, max 800px, JPEG q72) + preview_url_from()
- diary.py: Preview beim Bild-Upload, preview_url in media_items + cover_preview_url
  in Kalender-, Karten- und Listenabfragen
- forum.py: Preview in _save_upload(), foto_preview_url in Thread-Listen
- Frontend diary.js: cover_preview_url in Listenansicht, Mediengalerie, Kalender,
  Karten-Marker + Popup; onerror-Fallback auf Original
- Frontend forum.js: foto_preview_url in Thread-Karten-Thumbnails
- Admin: 'Previews generieren (Bestand)' Button → POST /admin/media/generate-previews

backend/static/js/pages/forum.js

@@ -250,7 +250,7 @@ window.Page_forum = (() => {
     const fotoHtml = t.foto_preview
       ? /\.(mp4|mov|webm|m4v|avi)$/i.test(t.foto_preview)
         ? `<div class="forum-card-thumb forum-card-thumb--video" style="display:flex;align-items:center;justify-content:center;background:var(--c-surface-2)">${UI.icon('video-camera')}</div>`
-        : `<img class="forum-card-thumb" src="${_esc(t.foto_preview)}" alt="" loading="lazy">`
+        : `<img class="forum-card-thumb" src="${_esc(t.foto_preview_url || t.foto_preview)}" alt="" loading="lazy" onerror="this.src='${_esc(t.foto_preview)}'">`
       : '';
 
     return `