refactor: address PR #851 review round 5 (ingest transport)

- _clear_vector_sync_state also nulls shutdown_event / scanner_wake_event on
  shutdown, symmetric with the stream/producer fields (the next startup rebinds
  them via _wire_vector_sync_state).
- Comment that the "DocumentTask" string subscript in LocalTransport is
  intentional (TYPE_CHECKING-only class; anyio ignores the runtime type arg).
- Move app.py's annotation-only IngestTransport / TaskProducer imports under
  TYPE_CHECKING (the module uses `from __future__ import annotations`), keeping
  only build_transport at runtime.

Refs: Deck #196

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

nextcloud_mcp_server/vector/queue/transport.py

@@ -153,6 +153,9 @@ class LocalTransport(IngestTransport):
     """
 
     def __init__(self, max_buffer_size: float):
+        # "DocumentTask" as a string (not the symbol): the class is
+        # TYPE_CHECKING-only here, and anyio ignores the runtime value of the
+        # type argument — so the string is intentional, not a typo.
         send_stream, receive_stream = anyio.create_memory_object_stream["DocumentTask"](
             max_buffer_size=max_buffer_size
         )