insert_chat_files() stored any caller-supplied file_id with no ownership
check, so a user could attach another user's file to their own chat and
then read it through the shared-chat access path in has_access_to_file().
Filter file_ids to those the caller owns, is admin for, or can read.
Also repairs an UnboundLocalError introduced in 260ead64d: the existing
duplicate-check referenced `session` before it was assigned (db=session),
so the function threw on every call and no chat_file rows were persisted.
Classic298
3f1c52e018