Add Forgejo metrics + upstream latency histogram to Fly proxy dashboard

- Enable Forgejo /metrics endpoint (app.ini [metrics] section)
- Add Alloy scrape target for Forgejo metrics on indri
- Add upstream_response_time histogram to Fly proxy Alloy config
- Replace single p95 panel with p50/p90/p99 + upstream breakdown
  filtered to forge.eblu.me host

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

fly/alloy.river

@@ -65,6 +65,15 @@ loki.process "nginx" {
     }
   }
 
+  stage.metrics {
+    metric.histogram {
+      name        = "flyio_nginx_upstream_response_time_seconds"
+      description = "Upstream (Forgejo) response time in seconds, excluding proxy overhead."
+      source      = "upstream_response_time"
+      buckets     = [0.005, 0.01, 0.025, 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, 10, 15, 20, 30, 45, 60]
+    }
+  }
+
   stage.metrics {
     metric.counter {
       name        = "flyio_nginx_http_response_bytes_total"