docs(vector): note tiers_sig extensibility, warn on dead-letter placeholder cleanup failure

Round-3 review nits on PR #920 (none blocking):
- escalation_tiers_signature: TODO noting future settings that can rescue a
  previously-terminal document (a toggleable llm tier, a raised oversize cap)
  should be folded into the signature so raising them auto-retries dead-letters.
- Terminal-path placeholder cleanup: a delete failure here is real Qdrant I/O,
  not control-flow -- log at warning (was debug) for observability. Non-fatal
  (the durable marker is already written).

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

nextcloud_mcp_server/vector/processor.py

@@ -1108,7 +1108,11 @@ async def _index_document(
                                 user_id=doc_task.user_id,
                             )
                         except Exception:
-                            logger.debug(
+                            # A real Qdrant I/O failure (not control-flow): warn so
+                            # it's observable. Non-fatal -- the durable dead-letter
+                            # marker is already written, so the leftover volatile
+                            # placeholder is merely redundant.
+                            logger.warning(
                                 "Could not delete placeholder for dead-lettered %s",
                                 doc_task.doc_id,
                                 exc_info=True,