Previously, attachments relied only on blob_id, which made it possible
to attach blobs across accounts by enumerating IDs. We now require both
blob_id and blob_key, add cross-account validation to prevent blob
reuse, and centralize the logic in a shared BlobOwnershipValidation
concern.
It also fixes a frontend bug where mixed-type action params (number +
string) were incorrectly dropped, causing attachment uploads to fail.
