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 Provisioning.md

@@ -21,7 +21,7 @@ committed.
 | `shared/prometheus/rules/llm-fleet.yml` | alert rules + recording rules |
 | `shared/grafana/provisioning/` | datasources + dashboards (provisioned tree) |
 | `shared/alloy/config.alloy` | the log pipeline: journal source → label promotion → Loki push |
-| `shared/blackbox/blackbox.yml` | prober modules (`http_2xx`) + the loopback bind |
+| `shared/blackbox/blackbox.yml` | prober modules (`http_2xx`); the listen bind lives on the unit's `Exec` |
 
 Targets and datasources reference **container names** (`grafana:3000`
 `loki:3100` — `shared/prometheus/prometheus.yml:52-61`), never