feat(overview): self-observability, config and the container image

Closes Phase 3 of the overview-dashboard plan (tasks 3.7-3.9).

Self-observability
- OverviewMetrics: an observable gauge read from the published snapshot at scrape
  time, so the metric and the page cannot disagree about what the dashboard
  believes, plus a sweep-duration histogram. Instrument names follow the family
  METRIC-CONVENTIONS spec (overview.instance.status / overview.poll.duration,
  unit s) rather than the plan's literal zb_ names; Prometheus renders them as
  overview_instance_status and overview_poll_duration_seconds. Labels are
  application/node/group - "node", not "instance", because Prometheus stamps its
  own instance label on every scraped series and a colliding metric label is
  silently renamed exported_instance. Instances never probed emit nothing: the
  enum's zero ordinal is Up, so a placeholder would publish the single most
  reassuring answer about a node nobody has heard from.
- Two readiness checks make the dashboard's own /health/ready mean something:
  registry-loaded (reports the counts it bound) and last-poll-cycle-completed,
  which catches the failure where Kestrel is fine, the page still renders, and
  every card on it is quietly frozen. The heartbeat is stamped by the sweep, not
  derived from the snapshot's timestamp - the poller publishes the registry
  before it probes anything, so a stuck poller would otherwise look healthy.
  No cycle yet is Degraded inside the grace window and Unhealthy past it, so a
  restart does not fail readiness for its first seconds and a poller that never
  started cannot hide behind "still starting".

Configuration
- appsettings.json ships tuning defaults with an EMPTY Applications list, so a
  deployment that supplies no registry fails at ConfigPreflight naming the key
  instead of booting into a dashboard of nothing.
- Three registries, because reachability genuinely differs: Development is the
  host-reachable subset (4 instances), Docker is the full 16-instance fleet by
  container DNS name. The rigs publish almost no per-node HTTP port to the host
  - OtOpcUa publishes none (only Traefik :9200, load-balanced across the central
  pair, so it cannot identify a node) and ScadaBridge keeps site :8084
  container-internal. Ports were established by probing the running rigs, not
  read off the compose files, because the two disagree: OtOpcUa site nodes serve
  health on :8080, not the :9000 their compose anchor implies.
- Real Serilog sinks, not the family's empty "Serilog": {}. AddZbSerilog drives
  sinks entirely from configuration and adds none of its own, so the empty
  section produces an app that logs NOWHERE - verified against the running
  HistorianGateway container, whose log stream is completely empty for exactly
  this reason. The HttpClient level override matters too: 16 instances on a 10s
  cadence emit several Information lines a second of pure probe noise.

Container
- HistorianGateway-pattern runtime-only image; the compose stack joins the three
  rig networks as external, so bringing the dashboard up or down cannot disturb
  a rig. The Dockerfile clears ASPNETCORE_HTTP_PORTS as well as ASPNETCORE_URLS:
  on .NET 8+ the base image declares its binding via the former, and clearing
  only the latter leaves Kestrel logging "Overriding address(es)" every boot.
  No healthcheck - aspnet:10.0 has neither curl nor wget, and adding a
  --healthcheck entry point to production code to satisfy Compose is not a
  trade worth making.

Tests
- BootTests boot the real Program.cs: every endpoint anonymous, the page
  rendering from cache while nothing it watches is reachable, the Meters
  allowlist proven by an end-to-end /metrics scrape, and both refuse-to-boot
  paths (missing registry, stale window inside the poll interval).
  The registry is supplied via environment variables, not
  ConfigureAppConfiguration, because ConfigPreflight runs before Build() - when
  the factory's callbacks are applied - and would not see them.

170 tests, 0 warnings. Verified live against the running rigs: 16 cards, 16
gauge series, page and gauge agreeing exactly.

ZB.MOM.WW.Overview/docker/.dockerignore

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