A document whose preview render fails for good (a PDF whose first page
renders blank) was queued by the boot-time backfill, refused, and queued
again at the next boot, without end.
The thumbnail job now keeps the failure with the document
(document_thumbnail_failures, migration 0374): the reason, the attempts
since the last success and when the newest failed. The backfill, both the
boot discovery and the per-account pass, leaves a document out after one
deterministic refusal (raster-failed, empty-render, pixel-cap) or three of
any other kind, until the document changes. A direct enqueue still renders
it, and a preview that is made clears the record.
