fix(health): suppress 404 when health monitor disabled + fix audit test assertion

- Sidebar.svelte: defer healthStore.refresh() until feature flags confirm
  health_monitor is enabled; skip entirely when disabled
- health.js: remove throw error from catch block — log and return null
  instead, preventing 'Uncaught (in promise)' on expected 404
- test_report_audit_immutability.py: fix mock assertions —
  audit_service uses logger.reason/reflect/explore, not logger.info
- HealthStore and Sidebar now produce zero network noise when
  FEATURES__HEALTH_MONITOR=false

backend/tests/services/clean_release/test_report_audit_immutability.py

@@ -128,11 +128,17 @@ def test_audit_hooks_emit_append_only_event_stream(mock_logger):
     audit_check_run("run-immut-1", "PASSED")
     audit_report("CCR-immut-1", "cand-immut-1")
 
-    assert mock_logger.info.call_count == 3
-    logged_messages = [call.args[0] for call in mock_logger.info.call_args_list]
-    assert logged_messages[0].startswith("[REASON]")
-    assert logged_messages[1].startswith("[REFLECT]")
-    assert logged_messages[2].startswith("[EXPLORE]")
+    assert mock_logger.reason.call_count == 1
+    assert mock_logger.reflect.call_count == 1
+    assert mock_logger.explore.call_count == 1
+
+    reason_msg = mock_logger.reason.call_args[0][0]
+    reflect_msg = mock_logger.reflect.call_args[0][0]
+    explore_msg = mock_logger.explore.call_args[0][0]
+
+    assert reason_msg.startswith("clean-release preparation")
+    assert reflect_msg.startswith("clean-release check_run")
+    assert explore_msg.startswith("clean-release report_id")
 
 
 # [/DEF:test_audit_hooks_emit_append_only_event_stream:Function]