fix(vector): make NATS status subscriber resilient at startup

Follow-up to PR #814 review.

NatsStatusSubscriber.run() called task_status.started() *after* the fallible
pull_subscribe, so a NATS broker that wasn't ready when the MCP server started
would crash the lifespan instead of retrying. Bus status is a non-critical
observability path, so:

- signal started() before the first subscribe (semantics: "loop is running",
  not "subscription succeeded");
- retry a failed subscribe with backoff instead of propagating;
- on a real fetch error (not an idle timeout) drop the subscription and
  re-subscribe rather than fetching against a possibly-dead handle.

Also anchor the _content_hash etag-threading TODO to the PR #814 review thread
so it is discoverable outside git blame.

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

nextcloud_mcp_server/vector/queue/nats.py

@@ -54,10 +54,10 @@ def _content_hash(task: DocumentTask) -> str:
     """etag is the change-detection token; fall back to modified_at when it is
     absent (e.g. deletes, or sources whose etag we don't thread through).
 
-    TODO(follow-up): thread etags for file / deck_card / news_item scans too
-    (only note scans pass etag today). Until then their JetStream Nats-Msg-Id
-    dedup keys off modified_at, which misses content changes that leave
-    modified_at unchanged (e.g. a file move/rename).
+    TODO(follow-up, PR #814 review): thread etags for file / deck_card /
+    news_item scans too (only note scans pass etag today). Until then their
+    JetStream Nats-Msg-Id dedup keys off modified_at, which misses content
+    changes that leave modified_at unchanged (e.g. a file move/rename).
     """
     return task.etag or str(task.modified_at)