Same 'change shared shape, miss pinned tests' trap as cfa4fb4 — the
prior commit updated only test_backfill_mixed_aggregate; three sibling
tests in the same file also pinned the old {enqueued,ok,regenerated}
shape without 'scanned' and CI bounced.
Two coupled problems, operator-flagged 2026-06-01: "missing thumbnails
but triggering backfill found nothing."
1. **Backfill UI was a black box.** `POST /api/thumbnails/backfill`
returned just `{celery_task_id}` and the admin card said
"Enqueued." with no counts. There was no way to tell whether the
scan found 0 candidates, 5000 candidates, or whether the worker
even picked up the task. "Found nothing" was indistinguishable
from a broken queue.
Fix: refactor the scan into a sync helper (`_run_backfill_scan`)
shared by the Celery task and the API endpoint. The API now runs
the scan in an executor and returns `{scanned, enqueued, ok,
regenerated}`. The actual thumbnail generation work still goes
to the thumbnail Celery queue per row via
`generate_thumbnail.delay()` — the scan itself is fast
(SELECT id+thumbnail_path + a file.stat() per row).
2. **`_thumb_is_valid` accepted header-only corrupt files.** The
magic-byte check passed for any 8-byte file starting with a JPEG
or PNG header, including empty/truncated/zero-pad files that
browsers render as broken. Backfill counted these as `ok` and
never regenerated.
Fix: also require file size ≥ MIN_THUMB_BYTES (256). Real
thumbnails are minimum ~2KB even on solid-color sources; header-
only corrupt files top out around 12 bytes. 256 is well above
the corrupt floor and well below any legitimate thumbnail.
Plus the admin card now shows the per-run counts instead of
"Enqueued.":
Scanned 5,432 · enqueued 3 (2 regenerated) · 5,429 ok
bvandeusen