Feed cover image URLs were built only from the item's slug and file
extension, so the URL never changed when the cover image itself
changed. Podcast clients that cache by URL would keep showing the old
cover after an update.
Add a '?ts=' query parameter using the already-computed
entityUpdatedAt value, matching the same cache-busting convention
already used elsewhere in the app (see the updatedAt comment in
LibraryItemController.updateCover).
Applies to all three feed types: single item, collection, and series.
Fixes#5306
