fix(vector): guard dead-letter on etag, harden marker filter

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>

tests/unit/vector/test_dead_letter.py

@@ -151,6 +151,7 @@ class TestIsDeadLettered:
         assert conds == {
             "doc_id": "520189",
             "doc_type": "file",
+            "is_placeholder": True,
             dl.DEAD_LETTER_KEY: True,
         }
         assert "user_id" not in conds