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/blackbox/blackbox.yml

@@ -0,0 +1,18 @@
+# Blackbox exporter modules — the honest answer to "is this HTTP service
+# answering?". A Prometheus scrape cannot express it (an HTML front page is a
+# 200 that fails the scrape protocol), which is why the old `services` and
+# `comfyui` jobs sat permanently red on `unsupported Content-Type`.
+# See serverAppConfigs-docs/Architecture/Stale Signal Audit.md §1.
+# Loopback bind: the image's default 0.0.0.0:9115 would ride the LAN on this
+# Network=host container. Prometheus (bridge) still reaches it — its traffic
+# arrives on the podman gateway interface, which is not loopback.
+network:
+  listen_address: 127.0.0.1:9115
+
+modules:
+  http_2xx:
+    prober: http
+    timeout: 10s
+    http:
+      preferred_ip_protocol: ip4
+      valid_status_codes: [] # default: 2xx + 3xx