refactor(worker): trim observability helper docstring; clarify test fake

- Collapse _init_worker_observability's docstring to one line; the WHY moves
  to a concise inline comment (per review).
- Note that _fake_settings.ingest_queue is unused by the helper (test realism).

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

tests/test_cli.py

@@ -339,7 +339,7 @@ def _fake_settings(**overrides):
     real Settings.__post_init__ validation/derivation.
     """
     base = dict(
-        ingest_queue="postgres",
+        ingest_queue="postgres",  # for realism / worker() gating; unused by the helper
         log_format="json",
         log_level="INFO",
         log_include_trace_context=True,