refactor: address PR #851 review round 2 (ingest transport)

- Rename the lifespan-local `transport` to `ingest_transport` in both paths so it
  no longer shadows the get_app(transport=...) HTTP-transport parameter.
- Log the memory backend selection in build_transport, symmetric with the
  postgres branch, so startup logs name the chosen ingest backend either way.
- Note in _wire_vector_sync_state why eviction_task_group is intentionally not
  set there (it only exists once the lifespan's task group is running).

Refs: Deck #196

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

nextcloud_mcp_server/vector/queue/transport.py

@@ -222,4 +222,5 @@ async def build_transport(settings: Settings) -> IngestTransport:
         logger.info("Ingest queue: postgres (procrastinate); worker drains it")
         return DistributedTransport(producer)
 
+    logger.info("Ingest queue: memory (in-process anyio stream + processor pool)")
     return LocalTransport(max_buffer_size=settings.vector_sync_queue_max_size)