fix: address PR #814 review + SonarCloud gate

SonarCloud:
- Resolve 6 S5332 hotspots (http→https in test fixture URLs).
- S6418: hoist the unauthenticated AsyncOpenAI placeholder to a named constant
  + NOSONAR (genuine non-secret; gateway ignores it when unauthenticated).
- Fix two reliability bugs: None-index guard in the gateway token-cache test
  (S2259) and float `> 0.0` instead of `!= 0.0` in the sentinel test (S1244).
- status.py idle path sleeps 0.1s instead of sleep(0) (S7491); NOSONAR on the
  protocol-required async no-await aclose() stubs (S7503).

Claude review:
- Remove three leftover debug print() calls in app.py (logger.info already
  covers them).
- payload_backfill: drop parsed_at from the backfilled-keys docstring (it is
  per-document state, not a deployment scalar); add a clean 404 precondition
  for BasicAuth deployments without an OAuth token verifier.
- status.py: task_status typed TaskStatus | None (drop type: ignore).
- nats.py: TODO to thread etags for file/deck/news; note etag default → None.
- factory: warn on unknown INGEST_BUS_URL scheme; raise ValueError instead of
  assert for the external-mode preconditions.
- docs/configuration.md: document the decomposition hook-point env vars + that
  nats-py ships core (lazy-imported) and external+bus uses two NATS connections.

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

nextcloud_mcp_server/vector/queue/status.py

@@ -134,7 +134,7 @@ class NatsStatusSubscriber:
         self,
         shutdown_event: anyio.Event,
         *,
-        task_status: TaskStatus = None,  # type: ignore[assignment]
+        task_status: TaskStatus | None = None,
     ) -> None:
         """Durable pull-consumer loop. Requires a live broker (integration)."""
         import anyio  # noqa: PLC0415
@@ -149,8 +149,9 @@ class NatsStatusSubscriber:
             try:
                 msgs = await sub.fetch(batch=16, timeout=5)
             except Exception:
-                # fetch timeout when idle — loop and re-check shutdown.
-                await anyio.sleep(0)
+                # fetch timeout when idle — brief pause before re-checking
+                # shutdown, avoiding a tight check-and-sleep spin on quiet tenants.
+                await anyio.sleep(0.1)
                 continue
             for msg in msgs:
                 self.handle_message(msg.subject, msg.data)