Use the storage layer when backfilling media file sizes

[javihgil]

Problem

The migration/update path calculates missing file sizes from the media URL instead of using the storage driver.

Current code:

• src/EntityManager/MediaManager.php:189 checks getFileSize()
• src/EntityManager/MediaManager.php:191 calls filesize($version->getUrl())

Impact

This is fragile for non-local storage:

• remote URLs may not work with filesize() at all
• storage backends are bypassed even though the same method already uses the storage driver to download files for SHA1 calculation
• migrations can leave versions without size information or fail unpredictably depending on the backend

Expected behavior

• obtain file size through the storage layer
• avoid direct filesystem assumptions when the version points to remote storage
• add coverage for at least one non-local storage scenario