[metadata] Fix cleanup scripts to check for dead images

vrobbler/apps/books/management/commands/cleanup_book_metadata.py

@@ -18,8 +18,17 @@ MISSING_ALL = [
     "publish_year",
 ]
 
+def _cover_missing_or_broken(book) -> bool:
+    if not bool(book.cover):
+        return True
+    try:
+        return not book.cover.storage.exists(book.cover.name)
+    except Exception:
+        return True
+
+
 MISSING_GROUPS = {
-    "cover": lambda b: not bool(b.cover),
+    "cover": _cover_missing_or_broken,
     "summary": lambda b: not b.summary,
     "isbn": lambda b: not b.isbn_13 and not b.isbn_10,
     "pages": lambda b: b.pages is None,