feat(infra): auto-discovery Prometheus + Traefik metrics

- Prometheus: add docker_sd_configs for auto-discovery via labels
- Prometheus: mount Docker socket for Docker SD
- Traefik: enable --metrics.prometheus with entrypoint/service labels
- Traefik: add prometheus.io/scrape=true label
- Observability: mount Docker socket for Prometheus container
- Bump hub submodule (Traefik metrics queries + sparkline charts)

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

infra/compose/observability.yml

@@ -55,6 +55,7 @@ services:
     volumes:
       - ../../infra/otel/prometheus.yml:/etc/prometheus/prometheus.yml:ro
       - /prometheus
+      - /var/run/docker.sock:/var/run/docker.sock:ro
 
   # ── Node Exporter (Host Metrics: CPU, RAM, Disk) ──
   node-exporter: