fix: use DelPrefix in upload avatar FlushCache to clear all cached sizes

FlushCache was using keyvalue.Del with the base key
(avatar_upload_{userID}) but the actual cache entries are stored with
size suffixes (avatar_upload_{userID}_{size}). The Del call targeted a
key that never existed, so cached avatars were never invalidated.

Switch to keyvalue.DelPrefix to delete all size variants at once,
matching the pattern the gravatar provider already uses correctly.

pkg/modules/avatar/upload/upload.go

@@ -42,7 +42,7 @@ const CacheKeyPrefix = "avatar_upload_"
 
 // FlushCache removes cached avatars for a user
 func (p *Provider) FlushCache(u *user.User) error {
-	return keyvalue.Del(CacheKeyPrefix + strconv.Itoa(int(u.ID)))
+	return keyvalue.DelPrefix(CacheKeyPrefix + strconv.Itoa(int(u.ID)) + "_")
 }
 
 // CachedAvatar represents a cached avatar with its content and mime type