fix(overview): P3 review fixes 003-005 - no redirects on probes, per-target publish so a hung instance cannot stale the rest, probe docs (scadaproj#48)

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

docs/plans/2026-07-22-overview-dashboard-design.md

@@ -78,7 +78,7 @@ ZB.MOM.WW.Overview/                      # plain directory in scadaproj — NOT
 
 All four family apps are first-class registry citizens; `HasActiveRole` is simply omitted for the two single-instance gateways (mxgw, HistorianGateway), which have no active/standby concept — their cards show status without the Active badge.
 
-- Every instance is probed the same way: GET `{BaseUrl}/health/ready`, plus `{BaseUrl}/health/active` when `HasActiveRole`. There is deliberately no per-instance probe-mode switch — one probe model, one status derivation.
+- Every instance is probed the same way: GET `{BaseUrl}/health/ready`, then `{BaseUrl}/health/active` whenever ready answered (single-instance gateways publish session counts there; `HasActiveRole` only gates the Active/Standby badge — Overview-005). There is deliberately no per-instance probe-mode switch — one probe model, one status derivation.
 - `PollIntervalSeconds`/`TimeoutSeconds`/`StaleAfterSeconds` are overridable per application and per instance (instance wins) for slow links or chatty pairs; `StaleAfterSeconds` must exceed `PollIntervalSeconds` (validated).
 - Bound + validated with `AddValidatedOptions<OverviewOptions, OverviewOptionsValidator>` (`ZB.MOM.WW.Configuration`): non-empty registry, absolute http(s) URLs, unique app/instance names, positive intervals, stale > poll. Malformed registry fails boot via `ConfigPreflight`, not the first poll.
 
@@ -112,7 +112,7 @@ One `OverviewPollerService : BackgroundService`:
 - Parses the canonical shape `{ status, totalDurationMs, entries: { name: { status, description, durationMs } } }`. Parsing lives in a small `ZbHealthReportClient` inside this app for v1; promoting it into a `ZB.MOM.WW.Health.Client` package is a deliberate later step, not a v1 dependency (recommendation: promote only when a second consumer appears).
 - Derived per-instance state:
   - **Up** — ready 200 + `"Healthy"`; **Degraded** — 200 + `"Degraded"`; **Down** — 503 / `"Unhealthy"`; **Unreachable** — timeout, refused, DNS, non-JSON (kept distinct from Down: it usually means network/VPN/registry-typo, not the app).
-  - **Active/Standby** — `/health/active` 200 → Active, 503 → Standby, error → Unknown (only when `HasActiveRole`).
+  - **Active/Standby** — `/health/active` 200 → Active, 503 → Standby, error → Unknown (badge shown only when `HasActiveRole`; redirects are not followed, so a 3xx → Unknown — Overview-003).
   - Plus: last poll UTC, latency ms, per-check entries, consecutive-failure count (state flips to Down/Unreachable only after **2** consecutive failures to avoid single-blip flapping; recovery is immediate).
 - **Cache-first rendering:** results land in an in-memory `OverviewSnapshotStore` (single immutable snapshot swapped atomically); an event notifies circuits to re-render. Page loads NEVER trigger a poll — they read the cached snapshot and render instantly; the poller timer is the only thing that does HTTP. No persistence — a restart repolls the world within one cadence.
 - **Staleness:** every instance snapshot carries `LastUpdatedUtc`. When the UI renders (or a periodic sweep runs), any instance whose snapshot age exceeds its effective `StaleAfterSeconds` is shown as **Stale (out of date/offline)** regardless of its last known status — this catches a stuck poller, a paused refresh, or a machine-slept dashboard, not just failed probes.