feat: add IngestTransport port for local/distributed ingest backends

Finish the hexagonal ports-&-adapters split started in #183. The producer side
already had a TaskProducer port + adapters, but the consumer side was
unabstracted and the INGEST_QUEUE selection leaked into a duplicated
`if use_postgres:` branch across both app.py lifespan paths.

Introduce an IngestTransport ABC (vector/queue/transport.py) that bundles the
producer with running (or not running) the in-process consumer pool, built by a
single build_transport() factory:

- LocalTransport (INGEST_QUEUE=memory): in-process anyio stream drained by an
  N-worker pool that run_consumers starts.
- DistributedTransport (INGEST_QUEUE=postgres): wraps ProcrastinateTaskProducer;
  run_consumers is a no-op because the consumer is the external `worker` role.

Both lifespan paths now call build_transport + _wire_vector_sync_state (new
helper that centralizes the app.state / module-singleton / browser-app writes) +
transport.run_consumers + transport.aclose(), with no INGEST_QUEUE branching and
no getattr drain probe. Adding a future backend (Redis/NATS/SQS) is one new
adapter + one build_transport arm, with no app.py or scanner change.

Preserves the single-tenant parallelism invariant (one shared multiplexed queue
+ N-worker pool, per-document not per-user dispatch) and documents it in
ADR-028. The worker CLI is unchanged (it is the external consumer).

Refs: Deck #196 (Deck #197 tracks the explicit parallelism regression test)

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

nextcloud_mcp_server/cli.py

@@ -325,6 +325,11 @@ def worker(concurrency: int | None):
         get_procrastinate_app,
     )
 
+    # This is the consumer side of the distributed (postgres) ingest backend.
+    # Unlike the in-process anyio pool, the worker talks to procrastinate's App
+    # directly (run_worker_async), so it does NOT go through IngestTransport —
+    # DistributedTransport.run_consumers is a deliberate no-op precisely because
+    # this separate process is the consumer (see vector/queue/transport.py).
     workers = concurrency or settings.vector_sync_processor_workers
     app = get_procrastinate_app()