refactor: address PR #851 review round 4 (ingest transport)

- Clear the module-singleton ingest references (task_producer,
  document_send_stream, document_receive_stream) on lifespan shutdown via a new
  _clear_vector_sync_state() helper, mirroring the eviction_task_group cleanup.
  Defense-in-depth so a late webhook (or a module-singleton integration test)
  can't touch a producer/stream backed by an already-closed resource.
- Add IngestTransport.backend_name ("memory"/"postgres") and use it in both
  lifespan log lines, removing the last settings.ingest_queue read from the
  background-sync setup — the lifespan no longer inspects the backend at all.
- Cover backend_name in the build_transport adapter-selection tests.

Refs: Deck #196

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

nextcloud_mcp_server/vector/queue/transport.py

@@ -86,6 +86,15 @@ class IngestTransport(abc.ABC):
     def producer(self) -> TaskProducer:
         """The :class:`TaskProducer` to wire into ``app.state`` / the scanner."""
 
+    @property
+    @abc.abstractmethod
+    def backend_name(self) -> str:
+        """Short backend identifier for logs/metrics (``memory``/``postgres``).
+
+        Lets the lifespan log which ingest backend is active without reading
+        ``settings.ingest_queue`` — the backend choice stays inside the transport.
+        """
+
     @property
     def send_stream(self) -> MemoryObjectSendStream[DocumentTask] | None:
         """Memory backend's raw send end; ``None`` for distributed backends.
@@ -156,6 +165,10 @@ class LocalTransport(IngestTransport):
     def producer(self) -> TaskProducer:
         return self._producer
 
+    @property
+    def backend_name(self) -> str:
+        return "memory"
+
     @property
     def send_stream(self) -> MemoryObjectSendStream[DocumentTask]:
         return self._send_stream
@@ -221,6 +234,10 @@ class DistributedTransport(IngestTransport):
     def producer(self) -> TaskProducer:
         return self._producer
 
+    @property
+    def backend_name(self) -> str:
+        return "postgres"
+
     async def aclose(self) -> None:
         await self._producer.drain()