fix(vector): clear dead-letter marker on delete, treat oversize as terminal

Addresses round-1 review on PR #920:
- Delete path now clears the file's dead-letter marker after
  release_document_for_user (whose principal-based filter misses the
  user-agnostic, principal-less marker), preventing orphan-marker accumulation
  for dead-lettered-then-deleted files.
- Oversize PDFs (rejected by the pre-parse size guard, no pipeline_tier stamped)
  are now treated as terminal regardless of failing_tier -- no tier can parse an
  oversize file -- so they dead-letter instead of falling to the legacy per-user
  mark on the inline path.
- Gate the success-path clear on a non-empty etag (an etag-less file can never
  have a marker, mirroring is_dead_lettered's early return).
- dead_letter.py: payload typed dict[str, Any] (CLAUDE.md).

Tests: oversize-terminal dead-letter and delete-path marker clear.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/dead_letter.py

@@ -30,6 +30,7 @@ lookup degrades to "process normally", a failed write is logged, not raised.
 import logging
 import time
 import uuid
+from typing import Any
 
 from qdrant_client import models
 from qdrant_client.models import FieldCondition, Filter, MatchValue, PointStruct
@@ -88,7 +89,7 @@ async def mark_dead_letter(
         settings = get_settings()
         dimension = get_embedding_service().get_dimension()
 
-        payload: dict = {
+        payload: dict[str, Any] = {
             "doc_id": doc_id,
             "doc_type": doc_type,
             "is_placeholder": True,