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.

shared/prometheus/prometheus.yml

@@ -60,20 +60,31 @@ scrape_configs:
       - targets: ["loki:3100"]
     metrics_path: "/metrics"
 
-  # ComfyUI (no native /metrics; probe the UI)
-  - job_name: "comfyui"
-    metrics_path: "/"
-    static_configs:
-      - targets: ["comfyui:8188"]
-
-  # Blackbox-style probes for service up/down (published LAN ports;
-  # gitea and comfyui are covered by their own jobs above)
+  # HTTP liveness, probed honestly through the blackbox exporter. Scraping a
+  # web root is NOT a health check — an HTML front page fails the scrape
+  # protocol (`unsupported Content-Type`), which is exactly why the old
+  # `services` and `comfyui` jobs were red forever beside healthy services
+  # (Stale Signal Audit §1). The prober is a host-netns container reached at
+  # the podman gateway name (HostAddr= on the prometheus unit); it in turn
+  # reaches both the host LAN IP and bridge container names. comfyui's UI probe
+  # lives here now — its own metrics-scrape job is retired, one owner per
+  # service.
   - job_name: "services"
-    metrics_path: "/"
+    metrics_path: /probe
+    params:
+      module: [http_2xx]
     static_configs:
       - targets:
-          - "192.168.0.143:8081"   # SearXNG (via the Gluetun tunnel)
-          - "192.168.0.143:3005"   # OpenWebUI
+          - "http://192.168.0.143:8081"   # SearXNG (via the Gluetun tunnel)
+          - "http://192.168.0.143:3005"   # OpenWebUI
+          - "http://comfyui:8188"         # ComfyUI UI (gfx1151 generator)
+    relabel_configs:
+      - source_labels: [__address__]
+        target_label: __param_target
+      - source_labels: [__param_target]
+        target_label: instance
+      - target_label: __address__
+        replacement: "host.containers.internal:9115"
 
   # LLM fleet. Scraped on the LAN addresses where the endpoint is reachable from
   # this container, not the guarded vhosts: these are machine clients and