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>

tests/unit/vector/test_ingest_transport.py

@@ -44,6 +44,7 @@ class TestBuildTransport:
 
         assert isinstance(transport, LocalTransport)
         assert isinstance(transport.producer, MemoryTaskProducer)
+        assert transport.backend_name == "memory"
         # Memory backend exposes both raw stream ends.
         assert transport.send_stream is not None
         assert transport.receive_stream is not None
@@ -61,6 +62,7 @@ class TestBuildTransport:
 
         assert isinstance(transport, DistributedTransport)
         assert transport.producer is producer
+        assert transport.backend_name == "postgres"
         # Schema applied once on the open pool before any defer.
         producer.ensure_schema.assert_awaited_once()
         # No in-process stream for the distributed backend.