Replace flat UUID filenames with a browsable, dated layout (spec.md item 9):
<STORAGE_DIR>/<YYYY>/<MM>_<DD>_<dollars>.<cents><ext>, from the receipt date
and amount. Same date+amount collisions get a _1, _2, … suffix via exclusive
create (race-safe). New uploads only; serving is unaffected (blob-backed).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>