fix(vector): address PR review round 4 — backfill resilience + degraded-mode docs

- Remove three stale `# Use numeric file ID` / `# Pass file path` comments
  in scanner.py. file_id is already normalized to str() above each call
  site, so the inline comments mislead readers.
- Wrap `_backfill_doc_id_to_string` scroll loop + sentinel upsert in
  try/except Exception. The qdrant_client singleton is assigned before
  this migration runs, so a transient scroll failure was leaving the
  process holding a usable client with int payloads permanently
  unbackfilled until the next restart. Catch broadly, log ERROR with
  exc_info, and return without writing the sentinel — next process
  restart retries from scratch.
- Note `:memory:` mode behavior near the sentinel constants so future
  readers don't read the every-start scroll as a bug.
- Document the two degraded-migration ERROR log signals in
  docs/configuration.md so operators know when a clean restart is
  required to recover indexing.
- Add unit test asserting scroll-time exceptions are logged and swallowed
  without writing the sentinel.

Closes round-4 review feedback on PR #773.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

nextcloud_mcp_server/vector/scanner.py

@@ -484,11 +484,11 @@ async def scan_user_documents(
                     await send_stream.send(
                         DocumentTask(
                             user_id=user_id,
-                            doc_id=file_id,  # Use numeric file ID
+                            doc_id=file_id,
                             doc_type="file",
                             operation="index",
                             modified_at=modified_at,
-                            file_path=file_path,  # Pass file path for content retrieval
+                            file_path=file_path,
                         )
                     )
                     file_queued += 1
@@ -547,11 +547,11 @@ async def scan_user_documents(
                         await send_stream.send(
                             DocumentTask(
                                 user_id=user_id,
-                                doc_id=file_id,  # Use numeric file ID
+                                doc_id=file_id,
                                 doc_type="file",
                                 operation="index",
                                 modified_at=modified_at,
-                                file_path=file_path,  # Pass file path for content retrieval
+                                file_path=file_path,
                             )
                         )
                         file_queued += 1
@@ -581,7 +581,7 @@ async def scan_user_documents(
                                 await send_stream.send(
                                     DocumentTask(
                                         user_id=user_id,
-                                        doc_id=file_id,  # Use numeric file ID
+                                        doc_id=file_id,
                                         doc_type="file",
                                         operation="delete",
                                         modified_at=0,