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/media_utils.py

@@ -157,6 +157,41 @@ def convert_media(data: bytes, filename: str) -> Tuple[bytes, str]:
     return data, ext or ".bin"
 
 
+_PREVIEW_EXTS = {".jpg", ".jpeg", ".png", ".webp", ".gif", ".heic", ".heif"}
+_PREVIEW_MAX  = 800   # längste Seite in Pixeln
+
+
+def generate_preview(data: bytes, ext: str) -> bytes | None:
+    """Erzeugt ein JPEG-Preview (max _PREVIEW_MAX px). Gibt None zurück bei Videos/PDFs/Fehler."""
+    if ext.lower() not in _PREVIEW_EXTS:
+        return None
+    try:
+        from PIL import Image, ImageOps
+        img = Image.open(io.BytesIO(data))
+        img = ImageOps.exif_transpose(img)
+        img = img.convert("RGB")
+        img.thumbnail((_PREVIEW_MAX, _PREVIEW_MAX), Image.LANCZOS)
+        buf = io.BytesIO()
+        img.save(buf, format="JPEG", quality=72, optimize=True)
+        return buf.getvalue()
+    except Exception:
+        return None
+
+
+def preview_url_from(url: str | None) -> str | None:
+    """Leitet die Preview-URL aus der Original-URL ab (fügt _preview vor Extension ein).
+    Gibt None für Videos, PDFs, bereits generierte Previews und leere URLs zurück."""
+    if not url:
+        return None
+    low = url.lower()
+    if any(low.endswith(s) for s in (
+        ".mp4", ".webm", ".mov", ".avi", ".pdf", "_thumb.jpg", "_preview.jpg"
+    )):
+        return None
+    base, _ = os.path.splitext(url)
+    return base + "_preview.jpg"
+
+
 def extract_video_thumb(video_path: str) -> str | None:
     """Extract a frame at ~1s from video_path and save as <basename>_thumb.jpg.
     Returns the thumb path on success, None on failure."""