feat(observability): blackbox exporter replaces the HTML-probe jobs

Prometheus 3 refuses HTML scrape bodies, so the comfyui/services probe
jobs were structurally dead targets reading as outages. HTTP liveness
now rides a blackbox exporter (shared/blackbox/blackbox.yml, loopback
bind, http_2xx module); prometheus.yml probes SearXNG, OpenWebUI and
ComfyUI via __param_target relabeling. The unit is a boot-persistent,
self-healing root quadlet.

dankclaw/observability/quadlet/prometheus.container

@@ -45,6 +45,11 @@ Wants=network-online.target
 ContainerName=prometheus
 Image=docker.io/prom/prometheus:latest
 Network=dankclaw_default
+# Name the podman gateway so scrape jobs can reach host-netns processes (the
+# blackbox exporter listens on the host's loopback; a bridge container reaches
+# it at this address, never at 127.0.0.1). Same mechanism the hermes Caddy
+# upstream learned the hard way (Container Host Reachability in the vault).
+HostAddr=host.containers.internal=169.254.1.2
 User=0:0
 Exec=--config.file=/etc/prometheus/prometheus.yml --storage.tsdb.path=/prometheus --storage.tsdb.retention.time=30d --web.enable-lifecycle
 PublishPort=127.0.0.1:9090:9090