fix(grafana): repair invalid JSON in overview dashboard

overview.json was missing a comma between the "Health Status" and
"App Error Rate (from logs)" panel objects, making the whole file
invalid JSON. Grafana's file provisioner skips dashboards that fail to
parse, so the entire "trails.cool Overview" dashboard (5 panels) never
loaded — a silent gap in observability. Validated with json.load.

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

infrastructure/grafana/dashboards/overview.json

@@ -137,7 +137,7 @@
           ]
         }
       }
-    }
+    },
     {
       "title": "App Error Rate (from logs)",
       "description": "Error-level log entries per second from Pino (level 50=error, 60=fatal). Captures crashes and unhandled errors that metrics may miss.",