feat: recreate thumbnails

[SeduroDotCom]

Madek/Madek#920

[urs-van-binsbergen]

Tested ✅

Little detail: typo in the commit comment in webapp:

Submodule webapp 94bd7bc..db117b0:
  > chore: udpate dl

[nimaai]

good. the only concerns:

1. Non-atomic recreation window (main risk): recreate_thumbnail_files! deletes the old Preview rows and their files before calling create_previews!. If
   create_previews! raises (as tested in "when thumbnail recreation fails"), the media file is left with zero previews until the action is retried successfully.
   For an admin tool this is probably an acceptable trade-off (old files must be removed to reuse filenames), but it's worth confirming that's intentional —
   e.g. document it in the flash/error message or the linked issue, since a transient conversion failure now destroys working thumbnails rather than leaving
   them in place.
2. Narrow rescue in delete_thumbnail_files: only Errno::ENOENT, Errno::EROFS are swallowed. Other IO errors (e.g. Errno::EACCES on permission-restricted
   storage) will propagate, delete no more files, but the preview rows for not-yet-processed files have already been deleted in the same transaction —
   consistent with point 1, just flagging the same class of issue surfaces here too.