Addresses @mkeller (Markus) — fixes(adr): "the ADR doesn't mention
in-memory BackfillStatus" and "treat this as a layering exception,
acknowledge it explicitly". Two new paragraphs under Operational caveats.
Refs #307
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Captures why thumbnails render in-process rather than being delegated
to ocr-service. Prevents a future reviewer from rehashing the decision
or moving it to the Python side without knowing the trade-offs.
Refs #307
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>