add diagnostic dashboard served from prometheus-exporter

- dashboard.html: standalone HTML, fetches :9090/metrics, auto-refresh
- Exporter serves dashboard at /dashboard and /dashboard.html
- Cards: Pipeline, Audio, Mixer, MIDI Bridge, LED Viz
- Live gauges, counters, service status dots, last-activity timestamps

scripts/prometheus-exporter.py

@@ -236,6 +236,9 @@ def format_metrics():
     return "\n".join(lines) + "\n"
 
 
+DASHBOARD_PATH = os.path.join(os.path.dirname(__file__), "dashboard.html")
+
+
 class Handler(http.server.BaseHTTPRequestHandler):
     def do_GET(self):
         if self.path == "/metrics":
@@ -252,6 +255,18 @@ class Handler(http.server.BaseHTTPRequestHandler):
             self.send_header("Content-Type", "application/json")
             self.end_headers()
             self.wfile.write(body)
+        elif self.path in ("/dashboard", "/dashboard.html"):
+            try:
+                with open(DASHBOARD_PATH) as f:
+                    body = f.read().encode()
+                self.send_response(200)
+                self.send_header("Content-Type", "text/html; charset=utf-8")
+                self.send_header("Content-Length", str(len(body)))
+                self.end_headers()
+                self.wfile.write(body)
+            except OSError:
+                self.send_response(404)
+                self.end_headers()
         else:
             self.send_response(404)
             self.end_headers()
@@ -268,6 +283,7 @@ def main():
     t.start()
     server = http.server.HTTPServer(("0.0.0.0", PORT), Handler)
     print(f"[metrics] Listening on :{PORT}/metrics")
+    print(f"[metrics] Dashboard: http://0.0.0.0:{PORT}/dashboard")
     server.serve_forever()