Add Prometheus metrics endpoint, Alertmanager, and 1h stats window

Add /metrics endpoint with Prometheus gauges for nodes, messages,
advertisements, telemetry, trace paths, events, and members. Include
per-node last_seen timestamps for alerting. Add Alertmanager service
to Docker Compose metrics profile with default blackhole receiver.
Add NodeNotSeen alert rule (48h threshold). Add 1h time window to
all windowed metrics alongside existing 24h/7d/30d windows.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -190,6 +190,25 @@ API_PORT=8000
 API_READ_KEY=
 API_ADMIN_KEY=
 
+# -------------------
+# Prometheus Metrics
+# -------------------
+# Prometheus metrics endpoint exposed at /metrics on the API service
+
+# Enable Prometheus metrics endpoint
+# Default: true
+METRICS_ENABLED=true
+
+# Seconds to cache metrics output (reduces database load)
+# Default: 60
+METRICS_CACHE_TTL=60
+
+# External Prometheus port (when using --profile metrics)
+PROMETHEUS_PORT=9090
+
+# External Alertmanager port (when using --profile metrics)
+ALERTMANAGER_PORT=9093
+
 # =============================================================================
 # WEB DASHBOARD SETTINGS
 # =============================================================================