The DELETE /api/personal/file disk-delete containment check used the
shared PERSONAL_UPLOADS_DIR root, so one admin could delete another
user's personal upload by passing its path (uploads are partitioned per
owner under <root>/<owner>/). Confine the check to the caller's own
per-owner subdir via _personal_upload_dir_for_owner(owner). RAG removal
and listing exclusion are unchanged (they still serve non-upload indexed
sources). Adds a regression test for the cross-owner case.
nopoz