fix: address PR #814 reviewer follow-ups

- gateway_client: guard token cache with a lazy anyio.Lock so concurrent
  embed calls share one M2M token request instead of racing
- status subscriber: distinguish idle fetch timeouts from real broker
  errors (log + 5s backoff) instead of swallowing all and spinning
- nats: warn when the bus URL uses unencrypted transport (non-tls://)
- collection_metadata: accept an optional shared httpx client, make TLS
  verify explicit, document the unauthenticated control-plane contract
- replace python -O-stripped asserts with explicit ValueError in the bus
  status builder and the api metadata source
- document why the nil-UUID sentinel point can't collide with content ids

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

nextcloud_mcp_server/app.py

@@ -361,8 +361,13 @@ async def _build_status_subscriber(
     """
     if not (settings.ingest_mode == "external" and settings.status_backend == "bus"):
         return None, None
-    assert settings.ingest_bus_url is not None
-    assert settings.tenant_id is not None
+    # Defence-in-depth (robust under ``python -O``, which strips asserts):
+    # __post_init__ already guarantees these when status_backend == "bus".
+    if settings.ingest_bus_url is None or settings.tenant_id is None:
+        raise ValueError(
+            "STATUS_BACKEND=bus requires INGEST_BUS_URL and TENANT_ID "
+            "(guaranteed by Settings validation)"
+        )
     store = StatusStore(max_size=settings.vector_sync_queue_max_size)
     subscriber = await NatsStatusSubscriber.connect(
         url=settings.ingest_bus_url,