video previews should be an array of thumbnails

lets normalize thumbnails into its own table. Images get a single thumbnail, videos get x (like 16).

```sql
create table thumbnail (
  id INTEGER PRIMARY KEY NOT NULL,
  media_file_id INTEGER NOT NULL,
  index INTEGER NOT NULL,
  image_data BLOB NOT NULL,
  timestamp INTEGER NOT NULL -- for media_type IMAGE, this is just zero.
);
```