fix(blackbox): unit had no Image and no Network=host - quadlet refused to generate it

The deploy died at 'Unit blackbox-exporter.service not found': a
.container without Image= is not generated at all, and the header
argued for Network=host it never declared. Completes the unit (digest
pinned prom/blackbox-exporter, Network=host) and fixes the bind
topology the first cut got wrong:

- Prometheus is bridge-networked and the netavark hairpin gap bars it
  from its own host's LAN IP, so a LAN-only prober bind is unscrapable
  and a loopback-only bind was claimed to ride the gateway on faith.
  The prober now binds 9115 on the podman gateway (scrape path) AND
  the LAN (healthcheck/diagnostics); the listen_address line leaves
  blackbox.yml so config and Exec cannot disagree.
- comfyui probe target http://comfyui:8188 -> http://127.0.0.1:8188:
  bridge container names do not resolve in the host netns; the
  published 0.0.0.0:8188 answers on loopback there.
- New seam: every observability unit must declare Image= (red on the
  shipped file by name); copier test pinned module presence instead
  of the removed listen line. Docs updated to the real topology.

serverAppConfigs-docs/Infrastructure/Observability.md

@@ -163,6 +163,6 @@ verify it through the APIs.
 - `scripts/gpu-exporter.py`, `scripts/gpu-exporter.service` (per-GPU host exporter)
 - `shared/prometheus/prometheus.yml`, `shared/prometheus/alertmanager.yml`, `shared/prometheus/rules/llm-fleet.yml`
 - `shared/grafana/provisioning/`, `shared/loki/loki-config.yaml`, `shared/alloy/config.alloy`
-- `shared/blackbox/blackbox.yml` (prober modules + loopback bind)
+- `shared/blackbox/blackbox.yml` (prober modules; the listen bind is on the unit's `Exec`)
 - `tests/observability_quadlet.bats` (boot persistence, no wildcard publish, probe addresses, delivery contract)
 - Host evidence 2026-10-18: `ss -ltnp`, `podman ps`, `systemctl --user is-enabled|is-active`, `curl` on `127.0.0.1:9090/api/v1/{targets,rules,alerts}`, `127.0.0.1:3100/loki/api/v1/labels`, `127.0.0.1:12345/metrics`, `127.0.0.1:3002/api/health`, `192.168.0.143:18093/health`, `journalctl --user -u signal-relay`