system/backend/backfill-sayings.mjs: document working --env-file run
The original "cd system && node ..." run command assumed system/.env
exists, but it doesn't — creds live in the vault. Updated the header
with the actual two --env-file invocation and a slim-install recipe
for the deps, after running the backfill end-to-end (307 mp3s
indexed, 25 jeffrey + 282 unknown-provider pre-stamping).