Addresses round-2 review on PR #920:
- Only dead-letter a terminal failure when the file has an etag to
content-address the marker; without one, fall back to the legacy per-user
placeholder mark (an etagless marker is unmatchable). + test.
- _dead_letter_filter now also matches is_placeholder=True (redundant with
dead_letter=True but lets Qdrant use the is_placeholder payload index).
- TODO(deck-349) documenting the dead-lettered-then-deleted orphan-marker leak
(out of scope; needs a marker sweep or TTL field) per reviewer.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
